Well I tried using an expandable foam in my new Aeroworks Edge 46-61 to hold the fuel tank and battery pack in place. I filled the area about 50% with the stuff and it expanded about half it's original size. It's called great stuff used for window and door frame insulation. If I have to remove the tank or battery all I have to do is cut or pull it out. I also poked holes into it (very carefully) to vent the battery for when I charge it.
This stuff is great and adds almost no weight.
If you try it make sure you leave room for it ti expand or it probly will push thru the structure.

Tom

I an not sure what brands are good. However, I tried this with a red can from Home Depot several years ago and the vibration of the airplane broke the foam down. The tank was not secure and there was dust.  Sig used to make an expandable foam but that was many years ago.  I do not know if they do today.  Interested in your results and what brand you used.

The brand I have is made by Dow and it's called Great Stuff.
It's in a blue colored can with a red label. It stays flexible and will expand and contract with temperature changes. I have not flown this plane yet but I have run the engine (91 surpass) tonight for about 15mins and it shows no signs of braking down.
I'll know better after a few flights.

Tom