A piece of PVC pipe a couple of feet long with caps glued onto both ends and a rubber grommet fitted into one end cap that will accommodate a brake booster check valve makes a good reservoir. Plumb a separate 3/8" or 1/2" vacuum hose to the brake booster and the vacuum trapped in the pipe will allow a couple of brake applications and/or a smooth shift of a 2-speed rear axle after the engine quits. A pipe up to about 4" in diameter fits nicely inside a frame rail under the cab.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!