Using a cart allows you to support the load in the middle so there’s not much weight pulling the bike out of balance. If the cart is a box on wheels you can put wetsuit, fin, mast base, extension etc in there. Having a board bag with a sturdy handle at the tip makes attachment easier but something can be worked out if you don’t have one.
The wheels at one end, strap to the seat thing is more simple but I suspect it might not work that great with the added weight of the rig. Also it might be tricky to rig up so the nose doesn’t hit anything under braking/turning. In fact the board might hit the rear wheel if you are short and have a low seat. Then again it might be just great. The simplicity of the idea is neat. I wanted something that works for my kayaks as well so it does’t really suit my needs but it could work well for windsurf kit. DIYing it should be really easy, and materials cost will be under $30, so I would never pay $130+ for it. Home improvement stores should have cart wheels, axles and webbing straps. You might be able to rig it up just by tying knots in the webbing, but if you want it to look a little more elegant, I’m sure you know somebody with a sewing machine. Just make sure the stitching is bombproof. Bar tacks or at least zigzag.