Rope Harnesses ^
Rubber/Leather Butt Plug Harnesses ^
There are a number of special purpose butt plug harnesses about. One supplier or these is Clone Zone (UK). There are a number of styles that are supplied that may contain an integrated butt plug/dildo. The dildo ones are nice because as you sit down your ass is freshly fucked, or your Owner can gently massage the back of your jeans to continue fucking you where ever he wants. These work well under a guys chastity shorts or his jeans.
Body Harnesses ^
Using a full body harness the leather strap that goes from the button of the cock ring between your legs to the adjustable strap at you back will keep the butt plug in. To stop the butt plug being remove when the Master is not there, it is usual to have a split ring (as in key rings) that connects to the end of the butt plug and for the leather strap to go through the center of the ring. With a number of harnesses they can have locking buckles so there is no way for the body to escape the embrace of the harness. These harnesses are available from Expectations. However there are many harness suppliers such as Expectations, CloneZone, Mr S, Mr B etc.