They are not air tight like you think.
You can DIY a silicone gasket into the lid -
Remove existing gasket - unless it's already doing the job - if it ain't broke...
Put release film on bucket rim.
Squirt silicone of choice around the gasket trough in the lid (I liked the white marine-grade stuff, it stays flexible longer).
--one layer, cut the tip to fill the lid trough about 2/3
Make sure you put the release film on the bucket - I didn't once...
Press the lid down firmly, but DO NOT SNAP SHUT.
Place a cinder block on the lid.
Come back tomorrow....
Seriously, beat it! Go smoke a bowl.
24 hours later - not first thing in the AM ...open bucket
remove film
Close & SNAP lid shut - should be kinda difficult the first few times.
With the lid ON - Wash bucket THOUROUGHLY with soap & water, rinse
Watch for bubbles actively forming --- bubbles mean you jacked it up. No bubbles?
Thoroughly dry the bucket - especially the lid and area around it - DRY.
Open lid, INSPECT for moisture - None?
Fill bucket with smelly product of your choice.
Let your buddie's (or your) dog or cat sniff it - most of them like the smell, so they'll alert if they get it.
All clear - just don't open it en route.
That's what we ran a few pounds a month in way back in the '80s. Never got pulled over, so I guess I never put it to the test...
There's easier ways - like a Pelican case - or seattle sports dry bag - ask any roadie or studio grip - they might ask the engineer for you.