Greets!

While mass loading via double wall construction yields increased rigidity, bracing with plywood, board or large dowel scraps gains it cheaper and lighter with the latter a big deal if you ever want to sell or move them up/down stairs. There's tons of threads here and on other forums on bracing as well as various ways to deal with the horn flares, so Google, etc..

The vent is optimized for prosound apps, so normally needs to be tuned a lot lower for HIFI apps, but since you want to use a sub system, best to seal them up tight as a drum and use an 80 Hz /4th order XO level matched via bi-amping.

PL400 has worked well for me since it came out, but many DIYers swear by Gorilla, just follow the instructions as Brad noted or you can wind up with a serious mess.

If you really want to make them heavier, just toss some 50# bags of sterilized sand in the bottoms, it won't affect box tuning enough to matter in this case.

GM