Sintra looks to be the same class of material as the Forex classic we use.
If so, then peel off protective packaging release film (to generate lots of static:-), spray with isopropyl alcohol (IPA), to remove surface grease and static . Wipe dry with lint free cloth.
Prior to coating, before sintra is at the nip of the roller, wipe across with tac cloth, and keep wiping as it is fed into the machine.
For bonding the two parts, you could apply adhesive (Mediatac or multitac) to the .5 foam. Peel back a couple of inches, line up the boards, and pull back the liner, to let them bond. Might be easier to use a heavier sintra - 24x36, I'd want at least 5mm. For added rigidity, apply a subframe to the back.
I think either mediatac or multitac would work - very useful to have a direct contact with Drytac - they may be able to supply a couple of 5m rolls for you to make a decision on.