Hi @Kohlrabi,
Thanks for joining the forum and great question! Since you mentioned you’ve already seen other threads about more general gym masks, I assume you’ve seen this already, but I’ll include it again just in case:
Fitting something under a fencing mask would be quite difficult I imagine, and I’m surprised you’re able to wear a FloMask - even if the fencing mask is up-sized! Have you looked into something like ReadiMask? I have a feeling it might be the best alternative: https://readimask.com
The biggest potential issue with it is mask collapse, but I think this could be overcome if it’s fitted without too much give in the filter material.
Have you tried any masks other than FloMask? If so, what were your experiences? It might give a me a bit more insight for further suggestions.