This subject seems to be rattling across at least two threads and has come up numerous times before. The fact is, very few seat/bed conversions were built to 'crash' standards and those that were, possibly wouldn't meet modern standards - note I said 'possibly'. I fully appreciate the concerns of parents when it comes to securing their off spring in the rear of the bus as I was in that position myself. I built two wooden rear bed/seats which whilst serving their purpose had inherent weaknesses and even if a seat belt fixing is sound, if the seat can give way under serious deceleration, you introduce a whole new set of problems, more so when the vehicle is loaded with stuff above the engine that can push on the back of the seat .
Various ideas and designs have appeared over the years for more robust rear bed/seats but at the end of the day its about having a seat that is firmly fixed (when in the seat position), and properly anchored, as I've mentioned before, with chunky bolts and spreader plates and belts also properly anchored. Thats as much as you can reasonably aim for. I chose to build my own, and one that I felt would absolutely meet those requirements, even if it meant a weight increase. In materials it cost well under £100 and I don't see how I could possibly improve upon it. I'm 17 stone (ish) and I can jump on that rear seat, and three heafty blokes couldn't tear it out with their bare hands (a good acid test for wooden seats). I am very happy to seat three children on it and it simply will not move. The rear bulkhead and floor offer good places to create fixings. For those choosing to make their own, the full width is 155cm and the seat should end up level (without boards or cushions) with the rear enging hatch level. The frame is very easy to make and hinges can be mounted or even welded to the sides of the frame as appropriate.
We can rattle on all day about what each thinks is safe but in the absence of a real standard, you fit what you personally feel confident with. Mine may be overkill in terms of guage of steel etc but its not made of recycled bunkbeds or reliant on screws in chipboard or plywood or farty 'L' brackets. In total, it does add the weight of one person but at the same time it occupies less space than a series of boards and bracing pieces. I can also remove it if I wanted to. As a parent I have absolute confidence in it - unlike the previous wooden ones I built. There are six potential seat anchor points although I found four were plenty. Sorry, don't have pic with base and back at mo but you get the idea.
Tallulah43 - if you want all the exact measurements etc, happy to supply. We can make them, but they don't work out as cheap that way, but your welder should be able to replicate what we did. The two uprights and the two front horizontal bars are VERY thick box section - they DO NOT bend whatsoever. We used heavy stuff to maximise space and obviate need for bracing. The rest (ie feet and top of sides) is angle. Back horizontal piece wasn't essential but created more anchor points. Seat back and base can be made from heavy ply or even steel-framed - few choices there.
The front of the seat can be infilled with a board or made into a cupboard with doors for storage. I just happen to have a tank under mine.
