Which kind of foam to use underneath acrylic aquarium?

[philfreenode]

In this thread it was unanimous that foam should be underneath an acrylic tank between it and the flat level stand for that extra reassurance.

Now I need to know which kind, Rigid or squishy? I am guessing rigid because we do not want the bottom to bow -at all-. My 350 gallon tank will only be exerting about 1 psi on the foam as its 96x36.

Rigid

Squishy

Squishy also?

 

[seedubs1]

I will be using a yoga mat haha

 

[jsc]

I had my tank, same dimensions as yours, on rigid insulation board. It was set up for 6 years before I moved. It worked great.
Good luckk