All your fills are the same size...
And on boats made in the US after about 1985, that would make 'em all 1.25" (the hose barb on the whole fitting is 1.5"). You'll find a whole bunch of 'em if you just google "deck fill caps"...pick the one that matches the others on your boat cosmetically.
Or doesn't...it can be very handy to have a fuel fill cap that looks different from a pumpout cap, and fuel and pumpout caps that don't look like your water fill cap. Also a good idea to keep a spare cap or two aboard, 'cuz they do have a way of going overboard, especially pumpout caps 'cuz they can't have a chain.