Recently saved a few £ by making foams for the boys Lunchboxes.
Only foams I could find were £9.99 a set and needed 2 sets. That's before tyres and wheels.
Got 2 sheets of upholstery foam on eBay for £4 posted. Enough to do two sets of lunchbox wheels on one sheet. Might do the Clod as well but the tyres are a lot harder on that so it doesn't need them as bad.
You can cut this with a sharp Stanley or one of those extending snap off blades.
I taped the one in the pic but don't tape them now for the car.
Cut a strip just over 12inches long by 2inches wide. Foam is 1 inch thick.
Stuff it in, I don't tape it, just cut it a bit long and stuff it in.
Less tyre wander, stiffer sidewall. Slightly better handling.