If you want to make your own seat do at least use the proper plastic hooks rather than risk damage from some random bit of angle steel.
Inflatable Boat Seat Hooks/ Brackets, Free UK. Postage | eBay
I asked a similar question about 18mths ago after buying my Honwave as I wanted a second seat. In the end I spent the £85 and bought a proper one because....
The Honda ones are actually a decent bit of kit being plywood in a thick grp coating with non-slip surface. Most other makes really are just a bit of badly finished too thin plywood.
I thought it may help enhance any future sale keeping the outfit 100% original.
The Honwaves are such a bargain for the boat so looked at overall the seat cost is fine.