You amaze me, you use plain ole corrugated cardboard, I go to Hobby Lobby and buy Bristol, 3X4 matt board and yesterday Lahgton Prestige 100% cotton
card stock, business card thickness.
Then using drawings proceed to assemble a frame work that is straight, true and warp free. I can only do this if using a laser cut frame!
Please know, many here watch you work your magic my friend
.
Rick