I ran across an article featuring cup holder adapters and bought some to test. These have expandable bases - you put them in the existing holder, turn the top, and little arms come out and snug them up.

These are the three I bought:

Amazon Basics Expandable Car Cup Holder

Upgraded 64oz Large Cup Holder Expander for Car


And the winner, the Seven Sparta Upgraded Cup Holder Expander.

I like the Seven Sparta for several reasons. The base expands enough for a tight fit in the OEM holder; despite the little rubber tabs looking flimsy, between them and the rubber coaster, everything I put in it stayed upright; and it has a feature the others don’t – you can reposition the base for an offset fit. They did this so two would work in most side-by-side setups (like 4th Gens have) but I’ll only be using one so I moved the top back, away from the climate controls, as shown in the pics. Having items up around the climate controls doesn't bother me b/c I use the Auto setting exclusively and just change the temp, which is easily accessible even with the biggest cup.

This is the best solution I’ve found yet and for $25, I highly recommend it.

Note - these pics are before I did the offset adjustment.

No surprise that this big bastard doesn't move around:



And this is big enough to stay put too:



But I thought there was no way this would stay upright. It proved me wrong - stayed just like this for my entire 41-mile commute over back roads, interstates, and city streets.



The offset screw holes:



Base reinstalled with offset:



Final placement:



Update - 15Sep23: I've had this for a couple of weeks now and it works. Very happy with it and highly recommend if you're tired of whatever you put in the cup holder just flailing around everywhere.