You could probably substitute closed cell foam, as long as it's soft.

I have some closed cell, I'll give it a try.  I have a wedge style tank, should the pick-up line be at the center of the air intake or below it?

Thanks again,
Rob

Thanks Bill, but I think I'm a ways from inverted flight.  Centered the tank on the intake as suggested.  I haven't been able to get my time off to synchronize with calm weather...