I used alum tweet, a plywood box with WHITE Armacoat spray on bed liner (not a DIY item) and made a moisture proof hinge down lid for when not in use. Don't forget, bass response is going to suck since there are no or few boundaries as in a room. This can be made up for in a number of different ways. I used multiple high efficiency woofers to get the low freq SPL up to 100db. With the proper crossover, a 90db mid and tweeter this was just about right. It's still serving me well after 5 years in fairly wet Eugene, OR
The question is, would these materials work outside without getting destroyed. Live in the midwest, so there is a very extreme temp range here. Didn't figure I could use any kind of fabric dome tweeters, never tried to do an outdoor speaker yet, need something by the pool. I can mount them under the overhang, so they'll be somewhat protected and the bass will have something of a wall to reflect off of.
The poly and rubber would be fine. The alum might get some oxidation but it would be thin and shouldn't be a problem. The Armacoat is practically bullet proof but several coats of good latex would probably serve as well. I can attest that high temps can cause adhesives to lose their grip which is why I recommended a white exterior if it's going to get any direct sun. I'd take it down and store it during the cold months when poolside activities cease.