Our "go to" source for spring perches on the round track cars was plain old channel iron, available at any steel supply business. Pick a width that's close to the size spring leaf that's being used, and hole saw a radius to match the axle housing diameter. The perch can be made any length that's appropriate for the application. Ours were about 6 inches long, with 4 or 5 center bolt holes drilled into them. It made adjusting wheelbase a very simple process.
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!