I lube up the handle of a hairbrush. Nice and thick, just long enough to hit my prostate and feels great!
The handle part of a screwdriver. Look around, find the perfect size that is wide enough but (butt?) not too wide, one that's long enough to hit the prostate. Plus, when you're done with it, no one would fault a guy for having a "manly" screwdriver on his nightstand.