On the other hand, if you're just looking for a nice way to fit a Thompson to an existing bike [Harley 45-inch flathead WLA's were the most common American and Canadian two-wheelers in use during WWII] IMA has 'em offered as a separate item for around $75 each. They're available for the M1 Garand or .30 carbine as well. And, helpfully, they also offer canvas cases for the Tommyguns and their magazines, as well as parts and parts sets.
