Executive summary: 9 mm open rubber grommets are readily available in the UK and work well. 10 mm might be even better if you can find them.
I recently picked up a “for repair” Boss Bass OverDrive ODB-3 pedal for cheap. It actually works fine electrically: the only problem was mechanical. The rubber grommet on the pedal screw had deteriorated, as rubber does, and over time the lack of protection led to the thread of the thumb screw being damaged until it was eventually impossible to unscrew it.
That was easily fixed by sawing off the end of the thread using a grinding wheel (on my knock-off Dremel-alike from Lidl which I bought years ago and which still works great except for the fact that the rubberised finish is deteriorating. As rubber does.) so that I could remove it, and ordering a replacement thumb screw.
(As this pedal has a rather beaten-up look, I replaced it with a slightly chipped thumb screw from a tidier-looking pedal that will get the new one instead.)
You can buy specially marketed and marked-up replacement grommets too, but they’re just open wiring grommets, so if you know the size you can buy a big bag for pennies apiece and replace the decaying grommets on all your pedals. I didn’t know the size, and all the things I could find were written by Americans and referred to fractional inch sizes, and even then they didn’t seem to agree about whether it should be 3/8” or 1/2”.
So I measured the slot (5.5 mm × 12.7 mm), did a bit of maths, and worked out that it had a circumference of 31.7 mm, corresponding to a diameter of 10 mm.
The sizes of grommet that I could easily find were nominally 6, 9, and 12 mm, but actually slightly larger (6.4, 9.3, 12.3 mm). Those are really just metric equivalents of fractions of an inch (1/4”, 3/8”, 1/2”) with a little bit of breathing room.
I ordered some 9 mm grommets, which dropped onto the doormat a few days later. I tried to fit one straight away, but it cracked as I pushed it in. The problem was that it was so chilly outside that the rubber was still cold and inflexible. I ran one under some warm water to make it more supple and tried again, and it was easy this time.
9 mm fits well. There’s a little vertical play, but it covers the range of motion without moving. I think a 10 mm grommet might be even better, but you probably have to live in a more consistently metric country to find one of those easily, or you can order a bag from China if you’re not in a rush.