We always called them dumbbells in the trade, but I see them referred to as fillet rubbers and fillet balls here..
https://www.ecfibreglasssupplies.co....uct/wax-fillet .. Other suppliers are available.
Regardless of how you made your mouldtool for the tank, it is still easier to make an insert in the mould for the filler recess. It would have been difficult to split the mould over the recess and an untidy way to mould it with the flash line down the middle. With the recess as a separate component it will be far easier to release. Provided you remember to very gently taper the sides.
I think you will be better off machining an accurate part for this, rather than hoping that the filler will fit the Brownie bites carton. Certainly on an injection model, the filler has a peripheral O-ring that takes no prisoners when you go to fit it. and then there is a surface rubber seal which is equally precise in it's requirements to fit.
All you need is an accurate stepped disc, you can measure and add the internal radii in wax/plasticine after fixing it in the mould... those will become external radii on the moulding... Obviously.
Alternatively, you can buy "flush" fillers which actually mount to a surface (and so are slightly raised in reality) you would just need to drill a hole for the filler neck.
Fairly sure my Renthal sprocket is counterbored for the (Alloy) flange nuts.