Cryogenic Perlite make a wonderful light replacement for sand.

As for molds use blue styrofoam and RED TUCK tape or surgical tubing to make your boxes.
You can cut blue styrofoam with a utility knife and tape it together with TUCK tape. Damp concrete will not stick to either.
Surgical tubing is cool and actually better than tape because you can wind it around the box and it keeps everything very tight. Then you just remove the tubing and the mold falls right off your planter.

Try to incorporate handles in your work if you plan on moving them. This can be as simple as a embedded eye bolt that you can
hook into with a handle for lifting. or just grooves under the bottom that allow a couple loops of rope to be used to pick up the pot.
just an indentation on the sides that allow you to get a grip or a nub you can hold.

BTW there are plenty of cheap dollar store wheels out there as well. embed a couple of those in the bottom and you can wheel your pot.

Embed two nuts on each side and you could add a nice bent wood handle. Or cabinet handles, or drawer pulls or just a few twists of wire.Knot nylon rope and embed the knots leaving a nice loop exposed. there are dozens of ideas for making handling heavy pots a breeze.

Every time you make a larger pot give consideration to how you will move it full of plants.

What are you using for your hypertufa formula? Sand does make your pots a lot heavier than a peat moss and perlite recipe. If you're worried about strength you can throw in some reinforcing fibers you can purchase at your local masonry store.

For a mold, you may want to give this a try: Although it is just a one-time use, I use a lot of cardboard boxes as molds, and have good luck with the finished trough. The boxes give a less rigid look than a stable, re-usable form because the sides bow out, etc. when you are tamping the tufa mix in -- they look like they've been kicking around for a few decades :)(unless, of course, you want a perfectly symmetrical trough). Just wrap the outside with surgical tubing like Dave says or even nylon rope to stabilize the sides while you are making it.

Try filling the inner cardboard box with sand to stop it from bowing in.

Mercedes

Also, if you have recently made your bowls, you will find that they continue to get lighter for weeks and even months. A trough that I coouldn't lift when it was fresh out of the mould is now quite manageable. This is especially noticeable with the lighter recipes since there is very little left when the water dries out.