Filling tyres is an art. The best I ever have seen was a closed cell soft foam donut that fitted very snuggley in the tyre. But bullfrog runs on 10" wheelbarrow tyres and up to now no prblem whatsoever. No filling except an inner tyre.

Do not use PUR expanding foam from a can. It wil give a mess, and even if the wheel comes out in reasonable shape, the foam will crumble during use. Giving you an even bigger mess.