I just ordered the tomtoc carrying case on Amazon. Seems like it gives the Steam Deck enough breathing room for almost any case you have along with some additional accessory storage space!

TomToc makes some of my favorite accessories for the Switch, just picked up a bag of theirs recently after seeing a detailed review by Bob Wulff, a gaming YouTuber.

I picked up the slightly bigger one they built for Steam Deck for a little extra space, but they've made one specifically for the Switch. The difference is around $12 for the extra space.

I also use their hard shell cases for pro controller storage and with the steam bag it looks like I've got more than enough space for the switch, 2 controllers, a case full of games, an external battery pack, and any additional adapters/cables.