Taking the word "cheap" as a key. Why not mod and old surfboard?

You can use almost anything as a board, old wake boards work well, strong enough just to drilll holes through the deck, add some grip and off you go. Standard short boards work well too, just need to fill the area where the bolts go through so you don't crush the foam. I once thought rocker mattered, but when learning your not out in light winds so wake board rocker makes no differences.

You might need to block the rear screws with washers so the foil doesn't point downwards due to the rocker line. I built it up with epoxy filler. Mounted it too far back the first try so drilled more holes.

Agree with the old surfboard. I routered out a block section and glassed in some 10 mil marine ply and bolted the foil to that. Worked great to get started. Nose lift is essential. I put about 10 sessions on it and could ride both ways but not gybe at that point. I then made a Paipo and never looked back. The swing weight of a surfboard is a little off, and you will understand what I mean after you get up and riding for a bit. I also tried using an old twintip that someone gave me and though I could ride okay, it never felt right either. Once on the Paipo my riding improved dramatically and pretty quickly.