Hey guys, if you want better material other than cardboard although maybe not as sturdy, try those vinyl placemats from the dollar store, or plastic chopping mats (search ebay, dollar store).
If you go to those arts and crafts store or a hardware store, they might have those plasticy boards that are thicker. I don't know what the material is but it feels/looks like plastic.
Also, BALSA WOOD! Typical arts and craft wood that kids use for school projects etc. Or those cardboard sheets architects use to make house models are great and have awesome finish to them.
I don't know if this would work, but polycarbonate sheets! Those things people put on roofs for insulation I think? This may not be as accessible for most people but for those who have men who hoard things in the garage, they might have access to these hahaha (my uncle had some, no idea why).
Once you find a good material, its a matter of just cutting and fitting. I've been looking for alternatives to buying base shapers online because some of them are rip offs, and you don't really know the quality of it before you buy. If you make them yourself then I guess you can use what you prefer and get a feel of it before you make it.