As a former catapult industry leader i speak with experience in this matter,multiplex is just the name given to plywood elsewhere in the world,what you need is birch backed ply AAAA rated,this has no voids or plugs and is the safest and strongest available,marine ply has many voids so i personally wouldnt risk it,this stuff is hard to scource and very pricey which reflects the quality at £90.00 for a 8' x 4' sheet.
Fish knows what hes on about.