Transferring heat from the motor to the shell was discussed previously (albeit briefly)
I still think the cheapest easy way is to fill the cavity with aluminium foil (or tape) whilst avoiding the electrically conducting parts. There should be a great benefit from conducting the heat to the shell. In sheathed heating elements, magnesium oxide powder is used for both its electrical insulation and thermal conductivity properties, so if you can find a source of that it would probably be better. Or maybe you could use a combination of thermal paste and some metal to better fill the cavity. Braided copper wire would also work very well.
Many of the conductive EMI shielding materials are made of sponge with a metallised fabric or soft flexible wire covering the sponge, which sounds perhaps like what you were looking for, but I am not sure if they will conduct heat as well as the other methods.
Alternatively you could use some sort of phase change material (PCM) that is solid at low temperature but liquidifies at (say) 55°C or so. You can use paraffin or maybe even bee's wax.. https://en.m.wikipedia.org/wiki/Phase-change_material
That would need to be added into the hot motor casing, but would surely help stabilise the motor temperature. The idea is to store the heat energy generated during the (high current) climbs, while conducting the heat away to the case. It takes a lot of energy to change the state of a material (some more than others though). As long as the material is compatible with the motor and cover, and melts at a temperature below that of motor de-magnetisation it should help reduce the peak temperature at the motor, keeping the motor from damaging temperatures.
Using steel wool could introduce some corrosion issues over time, and that could introduce particles into moving parts, but you could reduce that with grease coating the steel wool. Or not worry about that..