.358 gas checks, I do not think, will be of any value on a .366" bullet - unless they are large enough when new (Hornady perhaps?), to fit the depth of the 9.3's grooves & then some. Undersized gas\checks will likely be of little value.

A simple card wad of approx. .368" diameter will work much better, but - it must be in contact with the bullet's base. You may have to make a punch to cut these. If you cannot obtain 9.3 gas checks, then consider using a card wad - again, if must be in contact with the bullet's base.

If using IMR Trail Boss powder, this will be easy loading to the base of a seated bullet with no need of a filler. V
VV powder, Tin Star may or may not also have use in such loads. Contact that powder maker for further info on it. If using faster burning, less bulky powders other than AA5744, then a filler between the bullet and powder of some sort will be needed. No filler is needed with AA5744, however, all others should be in contact with the powder.

If you keep the velocity below about 1,400fps, the bullets will most likely shoot very well without any gas check, if kept between 12 and 15 Brinell and under 25,000psi.

I suggest the 9.3 mould is meant for 9.3 gas checks, or if not, then perhaps re-sized .375 gas checks.