I would simply request hard acrylic moulds from your existing HA supplier (NHS?).
No need to specify hypo-allergenic. That word is basically meaningless. You will either react to something or you won’t. The only way is to try it.
The irritation may simply be poor fit/abrasion and not a true allergic reaction. If you get a decent mould plus the right sized vent to reduce sweating and moisture, I’m sure this would improve matters.
You could always just pay for one mould if necessary to trial the material rather than two to reduce the cost.