woofers in a truck bed

There are a lot of different ways to install subs in a truck bed.  I was wondering once the holes are cut in the bed and in the cab what the best way to make a gasket is?  Are there any finishing kits out there?  I read on a post elsewhere some used rubber tire tubing so the cab and bed could still flex.  My install will require a larger opening as the woofer will be mounted from inside the cab (fully open and slot ported box).

thanks

Find somewhere that sells camper tops in your area.  Tell them you need some boot material.   Or tell them what you are looking for.

https://www.mediatunes.com/papeenterprises/index.html