We drove to Chico, CA and had a Transfer Flow auxiliary tank installed at their facility. They did an absolutely exceptional job! They ship to multiple installation companies throughout the US, though.
We have a 2020 Chevy Silverado 3500 with a "canister-type" locking slide cover. The Transfer Flow 37 gallon tank fits PERFECTLY and works exceptionally well. It gives us 73 gallons of fuel capacity. Could not be happier !!