Butt plug harnesses are great!
I've tried one like Mr S Leather's version (although it was the Extreme Restraints brand) and either way, I just don't like that annoying cock ring and front strap in the way since it tended to just cause a lot of irritation and chaffing.
So, with a lot of time on my hands, and a plug in my butt, I found the (IMHO) best butt plug harness design to be one with an open "V" shaped front. Meo makes a rubber one like that, if I recall correctly, but I wanted a leather one with locking buckles as well.
Lo and behold! The Deluxe Butt Plug Harness from Purple Passion in NYC had (and still sells) exactly what I was looking for!
Add a "plug adapter" from Mr S or 665 Leather (search "Universal Butt Plug Adaptor" and you'll find a few brands), and you've got yourself a wonderful setup for strapping that butt plug securely in place!
I've owned a Purple Passion harness since 2013 (I had to replace it after a few years as it was too worn out from, uh, frequent use, but I don't regret purchasing the exact one again and I take better care of leather products these days.
Comfy, secure, effective, lockable, low profile for hiding under clothes, AND, should one be inclined to use a chastity device at the same time: well, you can!
On a scale of 1-10 stars, I rate their harness a solid: 13 out of 10 stars.
If anyone's reading this and on the fence about it, give Purple Passion's butt plug harness a shot!!!