You don't want anything that can hold moisture. Ideally you want a air gap below the tank (usually done by gluing strips of nylon or similar under the tank so water can't get between the strip and tank itself) but with the clearance mentioned it sounds like that may be an issue. It might be worth considering coating the tank in something in an attempt to stop corrosion. As for anchoring it in position, I would be inclined to squeeze a bit of sika down the sides in areas where you will be able to get a knife or similar later to cut it away if you need to remove the tank again. On a slightly different topic - fitted some gear to a 24 Ocean Angler for a client - wasn't too happy with Allison when the non anchored fuel tank slid back in heavy weather and snapped the through hull water pick up off . Lucky it had a decent bilge pump.