So, I have converted to 1×9. I have a plastic bashring on the outer and a 34t chainring on the middle. looking at my frame there is room where the granny ring used to be to fit something with the same outer diameter as my chainring – would keep chain from coming off all the way round.

does some such thing exist?

There was a very expensive one advertised on here a few months ago but I can't remember the name

The old MRP system did that and had two rollers as well. I have 1×9 on two bikes, but with a Blackburn Stinger on one and an E13 XC guide on the other. There are a couple of guides that go on the down tube and just provide 1-2cm of side guidance on the inside, they work well.

just bodge an old front mech on. chains don't come off half-way round the front ring

A cheap and very reliable solution:
http://www.jdcycles.co.uk/components/tandem-components–accessories/n-gear-jump-stop-chain-guide

Blackspire Blackguard is an 'inner bash'.
BBG also do them I think.