[daz]

Not sure if it helps but two little tips that I have found work quite well if you can't find a seal big enough and need to improve the fit.

Stretch the neck seal before trimming. Stick a cylinder through the neck for 48 hours, standard 12 or dumpy 12 if you really need to stretch it a lot. They will stretch over time so better to stretch it a bit before trimming.

If it still needs trimming the biggest problem is getting a nice neat cut all the way round with no nicks or scags where splits can start. Stick it back over a cylinder inside out so you can see the rings and then go round with a really sharp stanley knife. You should end up with a very neat cut and no nicks.

HTH
Daz