Which foam inserts on 4WD front wheels?

[Nige]

Chaps,

Last weekend I saw a few 4WD cars that had Schumacher 2WD stagger ribs stretched over their front wheels. They seemed to go well, so wondered what foam inserts (if any) to use as I'd like to give it a go? I've got a Tamiya DB01R.

Ta very much,
Nige