I recently purchased a Gamma Seal which adds a rugged, screw-on lid to 5-gallon buckets.
The Gamma Seal is made from high density polyethylene and it creates a leak-proof and airtight seal making 5-gallon buckets great for transporting fish.
Using a rubber mallet, you tap on the included adapter ring which includes a rubber gasket. With the adapter in place, you can screw in the cover and you're good to go. I haven't tried removing one of the adapters from a bucket and I'm not sure it is possible.
How well does it work?
In a word, great!
Once installed, you can hold a bucket completely upside down and it won't leak. If you've ever had a bucket of fish tip over in your car, you will appreciate the Gamma Seal.
I used to transport a bucket containing a very large (and mean) Vieja hartwegi to the Cichlid Classic and it worked perfectly.
How much and where do I buy it?
Gamma Seals are available at some Fleet/Farm stores, but it is easy to find them from internet retailers: