Plantinyum i made a few air pots too with it , it works good only problem is finding a bottom. Shovel n seeds got a good idea with styrofoam drains iv done that and pressure washed it out or you can use some 1 1/2 - 2 inch pvc . I didnt even think about light weight concrete now i might be trying this too haha .
yes , for a bottom i usually used a piece of the same material, or a nylon of some sorts, it works if you dont need to move the pots too often or at all. However, i find that when the plant roots itself into this pot you dont really need a bottom to it, at this point i just place them on the concreet without a bottom, they are so rooted that no soil comes out when watering.
I think about also doing a concreet pot with this material on the outside, it will make a nice texture to the pot.
For a bottom one could just do a concreete slab, with asquare ,circular or whatever shape he intends the pot to be, then place the vertical part on top of it. That would work well but will be a little tricky if it needs to be moved ,if its a really big one and filled with soil...