Personally I think you'll be much more satisfied with the silicone embossing mat than with a piece of fun foam.
If you are using embossing folders you would get erratic results with the fun foam because it changes character the more you use it. It never returns completely to its original condition. The silicone compresses only to a point and returns to the original shape.
If you want to use a die to emboss instead of cut, then you absolutely must have the silicone mat. The die will cut through both the paper and the fun foam. The silicone mat allows the die to press into the paper but not cut through.
As for plumbers gasket...it is less expensive than the silicone mat and I believe it is rubber. It may have a different thickness and/or a different amount of give. You would need to try it carefully in your machine to confirm that it will do what you want it to do without putting too much stress on your machine.
ETA...Spellbinders' tan embossing mats are two for $10 at Amazon. That's the kind that I use.
https://www.amazon.com/Spellbinders-...57394004&psc=1
Fun Foam or Something else for a embossing mat
