I just use a pencil case

Regular one like this but it has 2 elastic straps that hold it in place. Works great...

https://smile.amazon.com/dp/B001BXZ28K/ref=cm_sw_r_apan_glt_fabc_F9KK03GX9ZRCGP4AYV4G?psc=1

just to clarify, the case in the link above is on the larger side, think like a thick briefcase.

this one https://www.amazon.com/dp/B001BXZ28K would probably be more ideal for a small travel-size, single volca case. and if you search the reviews, you can see a lot of them say the volcas fit perfectly.