I've looked into building one myself. I was looking for better performance and just over all better desine. The thing is when your done it will cost you a lot more than the the $300.00
Your best bet is to look at buying a used one. I see them all the time form $100.00 - 200.00
If your just looking at building your own. I was going to make it the same way surf bords are made. You could make the pontoons with build in storage compartments. This would also lighten up the pontoons. But still your looking at some $$$ and if you mess up it's going to cost you well over $300.00
Any way good luck. .