I am wanting to put 4 5x5 mid-bass driver under the back seats of my SUV. The question is, will a mounting board be sufficient or should I build an enclosure that'll fit nicely under the back seat?

Probably would be a good idea to seal the front wave from the back some how.

yeah you'll definitely want some kind of enclosure. If it's going under your back seats, sealed is probably your option (as opposed to IB). Just having a mounting board will make them play free air, which is pretty much the worst alignment for bass response.

Thank you for the comments, I am actually going to put them into the B pillars so I leave room under the seats and it has a cleaner appearance. Luckily my car already has plenty of insulation... not second skin, but it does the trick for now.