Best product to put between sub and baffle?

I built a fibreglass box and had not planned on putting fibreglass where the sub mounts, but got a lil carried away! So now the mounting location is not perfectly flat.

In the past I have found that regular weather stripping flattens out and seems to degrade, is there something better?

Foam/rubber weather stripping would be good...

What about using a sound deadener, dynamat for example. Too stiff maybe?

Has anyone tried this and found it to work well?