If you have removable headlamp lenses, e.g. stock '32 or '34 headlamps, take a look at hidden units that mount to the reflector. Go to www.parrautomotive.com plug in part number PRR-8150. Turns your whole headlamp into the signal lamp, but is almost invisible.

If you don't have that option you may have to fab a solution. Take a piece of lens material (amber or "white") and make it any shape you want. Fab a small box to fit inside your shock mount to hold a dual filament socket (available from any auto parts house) vertically, and to mount the lense to (through the shock tower). Use silicon gasket seal to seal the lense.