The next easiest way is to get a quart of Rustoleum American Accents satin black, plus a small short nap roller from Home Depot and roll the paint on. This looks much better than the truck bed liner. The only downside is that it's a vinyl paint and, while it drys to touch in a few hours, it's damp underneath the dry outer skin for several weeks. So your drivers will stick to the enclosure and if you later remove them the gasket may be torn. Also, don't place anything heavy on top of the speaker for a month or so.

The best looking way (other than automotive quality painting) is to use a spray can of Rustoleum American Accents satin black. THe paint in the spray cans is solvent based rather than water based. The downside here is that you need to first apply a good primer or the mdf will suck up the thin, solvent based paint and the finish will not look good. I use two succesive coats of a 50/50 carpenters glue & water mixture with sanding between coats and afterward. Then three coats of the spray paint with no sanding and about 15 minutes between coats. This is what I did with the Pi speakers, and the finish is very nice - comparable to that of black-painted high end speakers.