I've done up a design. The PDF can be printed actual size in A0 sheet size at a commercial printer.
If I get around to doing my one it will be on a budget Expanded Poly Propolene surfboard which for me will be a sinker.
For two foot straps with front foot strap single only.
The rectangular piece should be in 4 or 5mm thick aluminium.
Drill all the plate holes out to 5mm, tap to M6 or your imperial equivalent. Drill 8mm end holes for the slot ends and jigsaw betwee, keeping 8mm width and 90mm between track centers.
Cut or file out the nut insertion holes.
Drill all the wood bottom holes out to 8mm. Fit M6 T-nuts into the back hole and front 2 holes from underneath and fill the insides up with melted wax.
Drill all the top wood holes out to 6mm.
The other pieces should be cut from 4mm plywood. The top ply should be clad with surfboard cloth top and bottom over glass fibre patches around the holes.
The bottom ply needs a few layers around the plate attachment area and out to another 20 mm or so, top and bottom sandwiched between surfboard cloth. Be sure to put some extra fibre thickness around the rear of the plate attachment which takes all the shock loading from the foil attachment plat if the foil hits the bottom at speed.
Redrill all the holes through after fibreglassing but take care not to damage the T-nut threads.
Fit short countersunk screws through the bottom plate into the aluminium piece on the 8 side holes and the 2 centre holes and do them up.
Line the top plate up with your board centreline. Google foilbox placement and figure out where the boxplate centre needs to be based on board length.
Drill all the holes through top to bottom.
Check alignment with bottom plate, you will need 5 long screws fo fix front and rear foot straps and a more robust leash attachment at the back hole.
Squeeze silicon sealant into all the holes and do up the last 5 screws through straps or board leash fixing clip, top plate and bottom plate, T-nuts or last two (tapped) holes of the bottom plate.
If you follow the instructions then the loadings will be spread out by the plates and you can fit to even a fragile or bendy board.