Weren't you saying somewhere that they cracked on another mower? I have been lucky enough to find good inserts on otherwise really munted wheels and swap them over. I guess with the high turnover of mowers at your end, you can't find enough.
Pity you don't have your own plastic press. Are there 3D printers available at reasonable cost that could churn some out?

Ahh, if only victa had kept producing the thumblatch catcher series, they would be in better shape today!