I've used the blue foam from Home Depot between the maple motor mounts in place of balsa on the last dozen or so planes I've built, BUT, I cap both the top and bottom of the mounts with 1/32" plywood. Engine runs have been excellent. I believe the foam acts like a shock absorber and "kills" vibration from getting to the tank. Usually, I use 3/8" square maple engine mounts. Sandwiching the maple mounts and foam between the plywood caps makes a very rigid crutch assembly.
Chris

I've used the blue foam from Home Depot between the maple motor mounts in place of balsa on the last dozen or so planes I've built, BUT, I cap both the top and bottom of the mounts with 1/32" plywood. Engine runs have been excellent. I believe the foam acts like a shock absorber and "kills" vibration from getting to the tank. Usually, I use 3/8" square maple engine mounts. Sandwiching the maple mounts and foam between the plywood caps makes a very rigid crutch assembly.
Chris

The intent behind using the foam was never based on potential weight savings, although it has turned out to be a side benefit. Initially, I used 1/8" x 3/8" maple glued up in a Warren Truss configuration between the engine mounting beams under the tank, again sandwiched between two layers of 1/32" plywood, and started having fuel foaming problems. This happened on three different models, all using this type crutch. I concluded it was too stiff and transmitting harmonic vibrations straight into the tank. The foam insert solved the problem. Balsa might have worked too, but I never tried it since even contest grade weighs more than the foam. The plywood doesn't significantly contribute to the weight since the maple is relieved down to 1/8" wide under the tank. The composite structure is rigid, light and easy to build flat...and eliminates F1.
Chris