I replaced the foam on mine with foam for aquarium pump filters. A very open cell foam. I got it from a pet place, trimmed it and stuck it on with double sided tape.
It was originally in block shape that I split to thinner section and these two bits covered the heater intake perfectly., butted the two together in the middle so no gaps. I bought a packet with a couple of foam blocks so have a spare for future replacement. They cost a couple of £ .
