The factory design calls for two or more foam strips to be installed on top of the fuel tank. The idea is to have the top skin, when installed, to be compressing those foam strips a bit to hold the tank down and tight against the bedding (I used RTV).
I'm not aware of any foam that is guaranteed to either not lose its resiliency, or, not decompose with age, and is sure to resist any aviation fuel that might get to it when a little is spilled when filling the tank. So, I opted to form thin strips of wood (steam and clamp to a forming jig) which were then lamintate to almose the final thickness. Then the final gap is closed with fuel resistant RTV.
After forming and then reclamping into epoxied laminates
Final shape after laminating back in the forming jig
