I would put 2x4 wood base for the whole greenhouse and them shim it level with blocks. Make the blocks of different thicknesses by the width of the 2x4 and attach them with screws or nails from the underside of the wood foundation. There will be gaps between the blocks fill some with insulation if you want. Maybe some gaps would be good for drainage on the low side.

The hardware store will have all kinds of anchors show them a picture of the shimmed wood frame and tell ask them to help you with a plan. Make sure no hardware will be in the way of the bottom of your greenhouse.

Then build your greenhouse on the frame and screw it to the wood.

This post was edited by rustico_2009 on Thu, Mar 14, 13 at 12:10

What ever size board goi with DO NOT USE PRESSURE TREATED! It will eat thru the aluminum in 2 to 3 years.

I'm going with 2 X 12's to get some more height.

Here is a link.

Here is a link that might be useful: 6 X 8 HFGH

If you use pressure treated lumber you just need to use sill foam gasket between the wood and aluminum.

I think what I am going to do is anchor a 2 x 4 to the concrete, then attach a 2 x 6 to it ( like a L), I can cut these on a angle to make the foundation level, and add a couple of angle brackets to the inside for a little extra support.