Yes a right pain in the arse !!!
You can find some links that have drawings on the side panels, and how they assembled the rr bed.
This is what I did, after I had a few drawings off the net and after phoning JK up and asking for proper drawings LMAO for the fitting. But to be fare they can not send proper drawings because they will differ slightly in all vans.
I screwed 2 bits of board together aprox 100mm wide on the hinge that folds out nearest the sliding door and cut it down, top and bottom to make up my bed length. Made mine ( 6'-0"LG ).
I then made 2 card board templates,
1, The shape of the back wheel arch, and one 3/4 along the width of the bus, shape of the In/side back panel behind the engine bay, because i wanted mine to be a 3/4 width bed.
Make the templates high, past the point of where they should be for when the bed is made into the seat.
This is the tricky bit..
Try then to clamp the 100mm wide board that you made up for the bed length in the flat out position to a long solid peace of wood with a heavy weight on it, so the solid peace of wood sits on the engine bay, propping up the other end with the bed hinge clamped, to make up the bed, and exact height.
You now should have the position of the holes you have to drill on the side panel.
Mark position onto your cardboard template.
Note.
You need to move in aprox 50mm - 80mm each side of the of the van to get the side hinges to work properly, if not they wont fold up or down. Try to make sense of the Just Kraps drawing and see if the holes line up to aprox there Measurements on your template. Mine didnt but they were only 20mm out
lol.
Make a wooden side panel from that template from scrap wood and drill holes.
Cut down to aprox size and angle to make up the seat side panel. Trial and error will get the angle of the seat right, Also make the wood fit exact to the base of the frame, not how they say to do it with a gap. This will give it more strength.
When you have got the right angle and hole positions for one side, Put the wooden temp on top your new peace of wood and cut out and drill to the shape of the 2 side panels.
The rest is easy.
Make sure you have JK small drawing! And a few good hours to fix, Took me 10 hours to figure it out, and no help..
Hope that helps ?
Paddy..