I have a Waltham full hunter watch case with no crystal fitted and am unsure what type is required.

Should it be a domed or flat crystal.

I've read that crystals for hunters are difficult to obtain and fit but just looking at it, it seems that an ordinary flat one would suit.

Any advise or comments would be appreciated thanks.

Most have a slight dome to them. The only considerations I see are that it has to allow clearance for the hands to pass without touching the glass and it has to allow the lid to close without contacting the crystal. Aesthetics are a personal choice.