Resurrecting this thread because I'm after something similar so thought I'd check if anyone has new suggestions, and also note what I've found.
Drifta PVC or Canvas bags $80-$90 depending on material.
https://www.drifta.com.au/product/drifta-jerry-can-holder-bag/
Not bad but I'd prefer lockable. Something like the galvanised sheet metal ones with hinged lid pictured earlier would be ideal.