for stp's- especially the peecock, you can make a regular pair of underwear into a "harness" while still allowing access to the shaft to pee quickly and easily.
just get a pair of mens briefs of whatever you prefer (tight fitting) without the pre-made front pouch/fly and cut a hole the size of your shaft in the front, then put the packer inside and pull it through. the underwear will hold the balls/stp part in position while allowing access to the shaft to pee. this should eliminate any accidents where fumbling for your packer could cause it to shift or fall out.