I'm big fan of butt plugs (they have interestingly positive effect on obedience :), but butt plug tails do not look good. They're in wrong place and the center strap just gets in the way.

So I designed tail holder, which can be put on center leather strap of my harness. It consists of two parts I designed in CAD (overlaid), 3D-printed and then hot-glued together. I bought some horsehair on eBay, tied it together with zipties and hot-glued it to the hole. Looks very nice.