I used black urethane roof sealant purchased at HD. It comes in a caulking tube type container and takes several days to cure. I used a putty knife to spread it. Stays pliable and sticks to any surface. I did this 10 years ago when I added 4 G5s. Be sure to wear several layers of nitrile gloves.
2003 34' Dolphin 5342, W22, UP, UPGBrake, F and R Track Bars, Rear IPD sway bar, Koni FSDs, Safe-T-Plus, Scan Gauge II.. 2004 Jeep Liberty, Blue-Ox Adventa..