For those without access to foam, certainly you can find the materials in a trash bin.

It is a great Topic and money saving too !!!

I build from corro and scrap ply. An airframe for a 48" span sets me back about Rs. 130 on materials. Landing gear and electronics extra

Its even cheaper for me...
All planes I built till now are literally made for free...
Only adhesive cost

Here's another one made from cola plastic bottle, tape, and cardboard.   If you already have the used electronics from an old plane, then it's free, otherwise buying new motr/ESC is not free.