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@Brommers
My original plan was to shift the existing 53t chainring onto the inside position and have a chainguard cut from a large bit of titanium sheet I had, and to bolt that in the outside chainring position. However, as was pointed it it's probably best not to use a chainring designed to be used with a front derailleur in this instance, so I bought a Miche 52t 135mm BCD chainring. One problem solved.
The second problem arose because it turned out I'd already used the large bit of titanium I was going to have the chainguard made out of. And it occurred to me that using titanium for a chainguard is overkill. So I thought that instead I'd get one laser or water cut from a sheet of ali. However, SWMBO pointed out that there are already bits of ali cut into roughly the right shape which are commercially available. They're called chainrings, and all I'd have to do is machine the teeth off a nice big chainring.
So I also got a 61t TA chainring. Unfortunately it was a bit big to fit in the lathe (it'd fit on the faceplate but some weird and wonderful tooling would be required to actually turn it down) so instead I used the milling machine and a rotary table:
It was a bit of a swine getting the chainring true on the rotary table, but in the end I decided that run-out of 0.15mm was close enough. I had intended to put a radius on the outside edge using a radius cutter in the milling machine, but without making up a reasonably sturdy base plate to bolt the chainring down onto, it was impossible to keep it sufficiently flat to allow the radius cutter to give a decent finish. So instead I went old school and put the radius on the outer edge with files, emery paper and polish. The finish isn't perfect by any means, if you look closely, but anyone inspecting the chainguard on a Brompton closely probably needs to get a life. That includes me. Here it is in all its glory:
Next step, wheels. But that's going to require some engineering too, as my DIY wheel truing stand isn't really designed for 16" wheels...