Well I have some news for you, Squirrel my man. If you think the Mango harness would work well, just try a DIY version! Buy some elastic bands. I found some in the the sewing stuff area at Wal Mart. They're about $1 a pack, which is anywhere from 2-4yards depending on how wide a band you get. It is really flimsy stuff in a good way. You barely notice that it's there, yet it holds your junk amazingly well.
I use the small GV soft pack as well and this holds it awesomely, even a Soft Pack mini is held well. Though not personally used yet, I hear, and can see, how an STP would work perfect with this harness. Hope this helps!

Here are some videos to assist in the over look in use and the preparation which is very simple. If you can't sew, I just use some small safety pins.
and this one