Hello Robert. For that I suggest using butyl tape and non-sag sealant. For butyl tape I recommend the Alpha Systems Butyl Tape for RV Weatherproofing # AL28FJ. This is a 20' long roll of 1" thick tape.
For sealant I recommend the Alpha Systems 1010 Non-Sag Sealant for RVs # AL53KV. This is a single 10.3 oz tube non sag sealant and it is black in color. So, it will go perfectly with the Jensen # MSX60RVR marine speakers you were looking at.
I have linked our video for these speakers... view full answer...