Mik's advice is important. Frame mouldings are not made for outdoor use, so be sure to use a good hardwood and apply a few protective coats or polyurethane or acrylic over the factory finish.
Or, if you use an aluminum sectional frame, prepare to replace it in a few years, after the anodized finish fades. The license plates on my van are in Nielsen frames, and they look terrible after about three years. The cast-metal corner caps are pretty well obliterated, too.
I suggest using no paper or paperboard products in the framing. If I were you I would mastic the tiles to a piece of fluted polypropylene -- PVC sheet might be better -- and cover the back with a tight piece of the same plastic.
I suggest sealing the rabbet on the front with silicone sealant, too. Do not leave the rabbet exposed, as that would harbor insects and might a place for trapped moisture to make a mucky mess.