You have posted in wrong place!
However my P2 being a 39 slaoon is probably not identical but the straps are just flat steel bar shaped into curved section and bolted to other half which has just a L turn up for inside door post as pushes inside as door closes. Door post has 2 L tabs to bolt through too - really primitive but very effective. No pics at present though
Ply is the best stuff by far for backing trim but standard hardboard will do - coat with watered down PVA to waterproof
Do not undestand question as to door seal - any pics