As I've mentioned, I just use a padlock. I tie it up with cable ties so that it does not rock around. It doesn't look perfect, but it's covered by the spare wheel cover in any case.
I put it through one of the rim's stud holes, and through a hole in the backplate. Not sure if you spare weel carrier on the back door has the same holes. Probably not perfect, but it works for me.
In my photo, you'll also notice the original "locking" wheel stud. This appears to have been ground off by the previous owner, as it does not work...
Finally, I know - the rim looks a little bad. This was, however, the rim which I had on the back right of the 'trol when I had the run in with the Corolla a few weeks ago. As such, whilst everything looks fine (it's just got a coating of rubber from the other guy's tyres and a bit of red elsewhere from his vehicle) I'd prefer to have it checked out before using it day to day. In the meantime, I've organised another rim, which I'll get next week Friday. Once that happens, I can take this rim in to be assessed / have repaired. This may mean that there is a spare 4.5 GRX rim available at some point. Alternatively, I'm going to keep it as a second spare for a trip we're sort of planning into Northern Namibia next year.