To put it simply, it's actually easy to have one spawn, you just have to be patient and sometimes do other things so you don't miss your chance at having one spawn. Serebii has already listed the spawn chance, so I'll post a screenshot from there instead of typing it out:
Intense Sun, Snowstorm, and Thunderstorm will prevent a Space-Time Distortion from spawning and the timer will be skipped. There is a solution to that problem though, and it's so simple: Wild Pokemon.
That's right. If you are in a battle with a wild Pokemon, the timer for Space-Time Distortions will pause while the weather will keep continuing its cycle normally. Just wait long enough until the weather isn't one of the three undesirable ones anymore and then you can leave the battle, thus resuming the Space-Time Distortion timer once again.
Now, do not skip to any part of the in-game day, as doing so will completely reset the Space-Time Distortion timer. Going back to the village will do the same thing. Another thing to keep in mind is that you need to defeat the first Noble Pokemon before spawning Space-Time Distortions as they only spawn after the first Noble Pokemon (Kleavor) is defeated.
It's basically a waiting game to have one spawn, but by following this guide, you will be able to have more than one spawn after one another in a short span of time. Another thing, don't wait on the map screen, Pokedex screen, or inventory screen as they will pause everything, the weather and distortions included. To help make the wait a bit more bearable, you can set a physical timer that runs to 40 minutes. The camps are the safest places to wait. Don't forget to take a few seconds to check on the map from time to time though.
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