Gas Tank Mounting

Rhino Lined my tub and my whole tank a while ago and gonna put them back in, and I can't remember, but I think there was a rubber or something under the tank when I took it out. When I put my tank in, the spout touches the bottom of the hole in the side of the body and needs to come up 3/8". What is supposed to go between the tub and tank, and if you aren't sure, are there any ideas what I can/should use under there that is about 3/8" to 1/2" thick and is also weatherproof? Thanks guys.

 

http://www.earlycj5.com/forums/showthread.php?t=50227&highlight=gas+tank
and there are some more links in that thread about it.
I knew I had seen it b4 but it was way down the search list...