Agree with Fritz on this. I'm in the process (over next two days) of welding up a base out of box sextion that is similar to the Bluebird but will be boxed in underneath as seat houses an LPG tank. My original seat (MDF DIY) took some working out but in the end its down to where it sits in the flat position. In the end I had to make a 'test' seat which was only 2' in width out of scrap to get a feel for how the hinges worked and at what level the back and base board would lay once it was done. Once I'd done that, making a full sized one was much simpler.
In terms of commercial seats, the Bluebird has my vote in terms of its rigidity. I really don't like MDF or chipboard seats as they don't stand the test of time and I've never felt happy about putting a child on one, seat belts or not. Too many of them are fixed in with flimsy 'L' brackets and suchlike. Bolting through the bulkhead has to be a better idea.
I presume the lugs (those bits of metal sticking out at the sides of the front of the seat are meant to rest on brackets when the seat is down as thats another issue - having a firm front edge when bed is down.
As long as you're sure bed lays down at the correct angle and height, just drill your hole and fasten a mega washer or better still 1/8" plate with a hole drilled in it to strengthen fixing on the reverse. Theres nothing behind that bulkhead at the point you need to drill which is handy.
