FWIW, you could make your own templates by just multiplying the dimensions of pieces that you measure by whatever scaling factor you want. Once you have a "face" triangle, just lay it out edge-to-edge (sides, not bases) to draw it onto your material, allowing a bit of gap/space for folding. Three folds, tape, done. Use slots and tabs on the final, joined edges, if you want to be really slick.
Now, if you want THICKNESS, then you're gonna have to do a bit more math--basically, you need to build a pyramid in a pyramid, connected at their base edges somehow. I do not, however, believe this is a worthwhile thing to do unless you are scaling such that a large is, say, more than a foot tall. At smaller scales, your material thickness is likely to be more than sufficient for heft ("feel") and (un)stackability.
What material? I am starting to think camping mats could work well: use a V-shaped score along each folding edge, to make sharper edges and provide a
"trough" for your hot glue (which you should also mimic on the final, joined edges, for consistency). You can gt a variety of camp mat colors (i.e. no painting), and they are VERY kid-safe.
Anyhow, if you want Giant Mids, do like a Rabbit: go for and multiply! (heh, get it?)
David