If I let him use a litter tray inside, it would be no problem but what do I do? How do I collect urine once a day?
Hi Kris, there are a number of ways to catch a pee sample.
It's possible to buy non-absorbent litter from your vet and online. But it comes in tiny weeny quantities. Or, washed aquarium gravel would do the same job, possibly cheaper. Both can be rinsed, dried, and re-used.
In my experience crumpling clingfilm loosely over the cat litter is often a good way to catch a sample (the pee catches in the creases and you only need a drop or two.) But this technique tends to work better if the cat is already used to the litter tray.
Or...if you use clumping litter you may be able to just stick the end of the test strip into a freshly wet pee clump, if you can catch him in the act, or soon after.
With my new diabetic kitty I'm finding that just having a really shallow layer of cat litter seems to work quite well with her. It slows down the rate at which the pee gets absorbed, and there is sometimes a nice little pool on the bottom of the tray.
If your cat isn't too private about his toilet habits, you may, as some do here, be able to stick a spoon under your cat's bum while he's peeing.

Some people have also done this outdoors (much to the amusement of their neighbours, I expect), but you need to be able to pretty good at stalking your cat. If he tends to pee in the same place though, you could maybe wait in ambush...
Do you know if Rasmus will use a litter tray? Has he done so before? It may be that he relishes the opportunity to use it. In which case, that's good news (especially since you are moving house soon?)
But if he is reluctant to give it a go you could try to encourage him by sprinkling a little garden soil onto the tray (something that smells familiar). Or - and this is something you won't often see posted on a forum - if you know where he poops you can pop one of those onto the litter tray so that he associates one with t'other, just to give him the idea...
Given Rasmus is well over the 'renal threshold' it's likely he will want to pee a lot. So, hopefully you will have opportunities to catch some pee very soon.
Edited to add: Unless he's been diagnosed with ketones you shouldn't need to test the pee
that often unless he seems unwell or off his food. In which case testing is
very important. Occasional tests when you can get them should hopefully be fine (every week or so, for example in the beginning). If you find it fairly easy to do though then by all means test as much as you want to, it's always good to have the reassurance of a 'negative' result.
Anything above a 'trace' ketone result is a reason to talk to your vet ASAP. Ketones can get out of hand very quickly.
Eliz