The short answer

Get a hard case with a gasket, or a thick Mylar bag with a real zip seal. Those two cover almost everything. Silicone stash jars, thin baggies, and tins with a loose lid do not hold odor back, no matter what the listing said. If you buy one thing, buy something rigid with a rubber or silicone O-ring that compresses when the lid shuts.

Why the smell gets out

Cannabis odor comes from volatile organic compounds, mainly terpenes, that evaporate at room temperature. They keep turning into gas as long as the flower is intact. Any gap in the container is a path out, and porous materials like unlined fabric or soft plastic let those compounds soak in and re-release for weeks. I look at the seal first, the material second.

Containers that hold up

Hard cases with an O-ring

Pelican-style boxes, locking stash cases, and aluminum-lined carbon cases seal on a compressed gasket. That click you feel when you close it is the seal working. Downside: bulky, and the foam inside can pick up odor over time.

Carbon-lined bags

Activated carbon traps the compounds instead of just blocking them. These work well for short trips and for keeping one bag inside another. The carbon layer loses capacity after a while, often within a year of daily use, so expect to replace it.

Glass with a rubber gasket

Bail-top jars and latch jars with a replaceable rubber ring beat a plain screw-band jar for odor. Glass does not absorb anything, so it cleans out fully. Keep the flower in the jar, then put the jar in a case if you want a second layer.

What does not work

Sizing

An eighth fits in a small case about the size of a deck of cards. A quarter needs a slightly deeper box, and a full ounce wants something with a real latch. Oversized containers leave air space, and air carries odor. Match the box to the amount.

Keeping it working

Wipe the inside with isopropyl alcohol and let it dry with the lid open. Store an empty case with a spoonful of baking soda for a day to pull old smell out of the lining. Replace gaskets when they flatten or crack. A seal is a wear item, not a permanent feature.

Travel and the law

A smell proof container hides odor. It does not change legality, and in many places the odor of cannabis has been treated as a reason for a search. Rules vary by state and by airport, and federal law still applies at TSA checkpoints. Check the current rules for where you are and where you are going before you pack anything. If the container rides in a checked bag or a glovebox, know that heat softens gaskets and can cause a leak over a long drive.