For anyone reading this that missed or forgot about Doug's original thread (and the most successful homebrew cylinder so far in mopeds) I just bumped it, read through it have a look.
I reached out to Doug and he graced us with a pop in. Thanks Doug, should be interesting, I have already forgot how much I learned from your project.
I had completely forgot you tried and abandoned the 'center sprue' idea. Hmm perhaps I will have better luck with it.