I'm about to do the same thing with twin tanks.
glass in some new ply stringers covering them completely in resin (to water proof them) To box the tanks in giving about 50mm clearance all round, sit the tank on a small bit of foam in each corner to prevent direct contact with the hull.
mix and pour in some 2 Pac foam.
got mine here
Foams & Core Materials, 2 Part Polyurethane Foam Liquid, - CFS Fibreglass Supplies
this will fill up the sides and underneath the tank and bond it firmly in place.
then glass on a lid with an access door for the filler.
1. ensure you weight the tank before pouring foam to prevent tank lifting.
2. if the lid is ready before the pour you can quickly push it on and sit on it then the foam will cover top of tank aswell
3. make sure you mask of the business end of the tank to prevent foam covering connectors.
4.I will be installing a remote filler and breather to prevent the Risk of spillage being absorded by The the foam.
that's how I'm doing mine but other members may have a better solution.