Hi Duke - Junkyard projects are always worthwhile. I'd steer clear of epoxy granite as it will cost lots of $$$ which conflicts with one of your statements and I don't think at this level you will achieve what you think may happen with the EG. Sure its damp and its heavy (so what?) but its many $$$ and not very stiff and once cast you can't drill a hole through it easily to get that wire where you want it to go or attach that bracket you forgot about.. If your keen on casting use concrete grout from the hardware. 1/10 the price and just as good as EG.

Its great to have a project with your daughter. I would use good quality plywood and the bits & bobs you have. Ply is easy to work with and modify as you go along. Its stiff and damp and light which suits your concept. If you don't know what your doing with EG or grout it will be the same stiffness as high grade plywood and much less friendly. I have spent a lot of time working and testing EG and have yet to be convinced of its stiffness. I'd choose good ply over EG unless the part HAS to be cast.

Your expectations and aims are different to your childs. So spend as little $$$ as possible (as one of the project rules) , use plywood and the bits you have, get a machine working on a shoestring and see how it works out.... have fun most of all, another important rule. This build is not about creating a serious machine but a fun functional one. Once your through that exercise you can build a better one with this one etc.... Peter