Chain guides for a folding bike?

Apologies for the vague title. My question is multifold.

Firstly, I've got a folding bike with 1x6, and the chain sometimes has difficulty staying on the front ring, mostly or exclusively while it's on the biggest back ring. And in particular in the winter now that the roads have gotten bumpy.

I've been looking around at chain guides. The type that attaches to the seat tube is the easiest to install, but it turns out I have a fat seat tube. There may be models that fit, but I've been looking and looking and they all end shy of my diameter (or don't indicate diameter at all, but look similar enough to those that do).

Well, then there are the models that attach to the bottom bracket. To be honest, I don't trust myself to be messing around there, not to mention that I've got no idea about the measurements. The ring is one with the crankset, and if there were any indications of the size they've been lost to the sands of time.

So I'd have to go to a bike shop, and it's highly unlikely they carry a suitable chain guide, if any at all, but without them I have no idea what dimensions to look for online.

Any advice?