I have a custom built small form factor PC in a Dan Case A4 [0]. The case is a lot smaller than a Razer Core. And it's a whole PC, not just a GPU. Fits in a cabin bag or backpack and I have traveled with it.
You can go even smaller at 3.9L in a Velka 3 [1], if you use a single fan graphics card.
Buyer beware: for the Dan Case, it's only compatible with 2.25 slot (45mm) width cards at max. Most 3080s are 2.5 slot (50mm) wide. The EVGA 3080 XC3 Black is a notable exception, and is a very satisfying tight fit [0]. There's also a 3D printed GPU spacer to increase width of the case (sacrificing portability) [1].
I’d love to get a setup like this going with a 30x series card, but I’m not sure how difficult getting one at listing price is these days.