I thought 48mm was kind of the standard size for Chinese sets, actually. I know the automatic tables use the larger sized tiles at least.
Anyway, depending on where you live you should be able to find a tool case at pretty much any hardware store that you can repurpose as Mahjong Tile cases. You can also get foam inserts for cases like this that you can just cut to shape. Again, availability may vary depending on where you live but you're looking for something with foam on top, and a backing on it to you can layer the tiles into the box:
I actually recently made my own set of tiles out of casting resin so this was the solution I used for a case haha.