You can get great round molds that can be elongated and if you are VERY lucky you can get them free.
Does your city or any large company use plastic globes for large lights. These come in sizes up to 3 feet in diameter and if they get cracked...yep.. they must be replaced and the cracked ones go into the garbage.
Many cities and towns use these up to 2 feet in diameter and replace quite a few each year due to vandals and just cracking due to old age.
New they can cost in the hundreds of dollars. Cracked they go to land fills.
Check around you might get lucky.:)

Stumpie, Aluminum is not a good metal to use with concrete. It corrodes easily because of the alkilinty present. Get some metal lath used for stucco/plaster or get the 1/4 hardware cloth (actually a woven wire mesh) to get you're basic shape. I'd hate to see your project come apart from corrosion of the armature. Good luck.

I have never tried aluminum myself but Sherri Warner Hunter uses aluminum in her concrete works and recommends it in her books.
She uses aluminum rain gutter leaf guards which appears to be much like metal lath but easier to bend and shape.

I guess there are two camps on this subject. Reading on the professional concrete sites they pretty much say that unprotected aluminum embedded in concrete will corrode if exposed to moisture and cause problems.

So the key here is to make sure the aluminum is coated with something to protect it if the piece is going to get wet. Which means if it's going to an outdoor piece it's going to be exposed to moisture. Even out of the rain, the humidity will cause problems.

So make sure the aluminum is protected from the 'crete and you should be fine.

Here is a link that might be useful: Some info

Rockhewer is absolutely correct. No matter what Ms. Hunter says, you should never embed aluminum in concrete. The chemistry of these two materials is highly uncompatible, plus the expansion rates are quite dissimilar. Even when "coated" it performs very poorly and is problematic over time.

SCREENING ARMATURE

Steel or aluminum wire screening make excellent large scale armatures. Fiberglass screening, which is softer and less rigid, is fine for small objects. All are easily cut and shaped ;and held together by sewing or with twisted wire. Purchase them from hardware or building supply stores.

PROCEDURE FOR SCREENING

After creating your 3,D shape and mounting it on a board if small, begin the stiffening and space filling by

brushing or spraying on a Neat Mix of 1 part P-II. ¼ water and 1 to 2 parts Portland Cement. Let it dry. Next

add a heavier Neat layer containing 1 P-I!, ¼ water and 3 Portland which is brushed or spatulated on, to

achieve texture. If a smooth finish is desired, dip your tool into P-I! and then smooth. followed by dry and wet

sanding and/or tooling as previously described. Embedded objects can be added to the mix or stuck into the wet POLYADAM Cement.

A small object with many close folds may now be finished and can be painted. Larger objects and those with

free form extensions will require additional layers of POLYADAM Concrete. and reinforcing Medium-Light or

Heavy Scrim and possibly rebars or steel rods for structural strength. Simply build one layer of POLYADAM

Concrete with Scrim at a time until the object will support the anticipated stresses it may be subjected to. A

large wall piece will require less support than a free standing sculpture that kids will climb on.

From the POLYADAM website. Is POLYADAM different in some way from regular concrete?

ALUMINUM GUTTER GUARD AND WIRE LATH ARMATURES

Gutter guard is available in about 6" wide rolls in hardware stores and wire lath for plastering from building suppliers. Because they have a heavier metal construction they are easier than screening to shape into intricate

detailed shapes. Gutter guard is excellent for jewelry.

Here is a link that might be useful: Aluminum screening

Is POLYADAM different in some way than regular concrete?

I had type this question into the above post but for sme reason it did not show up.

Artists and professionals can often be at odds over details of using the same materials. I dont know about the Polyadam recipe but anything containing Portland cement is going to be highly alkiline. Alkilinity is as powerful as any acid. It's just at the other end of the PH scale.

It attacks the aluminum and as readily apparent, aluminum is not as strong as steel. Even using aluminum wire on steel rebar causes problems in two ways. The aluminum corrodes much faster than the steel. Also a galvanic corrosive process is initiated that accelerates the deterioation of the metals.

Aluminum powder is added to a cement mix to cause it to areate and increase in volume. This is simple chemistry. It's reaction that forms hydrogen bubbles. The same hydrogen bubbles form in concrete that has aluminum embedded in it, tho at a smaller scale. The hydrogen bubbles have nowhere to go but out and that's what causes the concrete to crack or spall off from the pressure.

If you want to make/build things with aluminum as as the armature you are most certainly welcome to do so. I'm going to follow the leads of the professionals and stay away from aluminum if I want my stuff to last.

Quote - "Gutter guard is excellent for jewelry". This about the only thing it may be good for. As I said in my first post if the piece is going to be exposed to moisture in any form including high humidity it's going to corrode and keep corroding until its gone. The gas bubbles cause concrete to crack and spall. It's just simple chemistry.

Sorry, I meant to include this before.

Here is a link that might be useful: nonferrous metals and concrete