It depends on the model of the filler you have. If the filler is a combo vent and fill, it is probably like a Perko model 541. or 540. I am not sure there is a locking cap for those models, you woyld have to replace the whole fill, not just thecap.
If you have a non vented fill ( there is a separate vent line and fitting , you have varied choices for locking caps.
Note that if you get a locking cap for a non vented fill but use it on a vented fill, you will probably break down at sea