I had a belt pouch and a 65L backpack. I can't use the belt pouch and the backpack belt on the same time. the backpack belt has no space to fix the pouch after it is tied.

So I disgned this adaptor to fix it on the water tube loop on the backpack strap.

If you want to fix it tighter. You can add an M3 strew and nut on it.

You can also modify the gap width depand on the thickness of the pouch strap. 

My belt pouch have thicker belt, due to there are snapped bottom on it. It will leave a big gap after the puch it fixed . So I add a clip-on phoneholder to fill the gap.(for Sony xperia 10IV)

If you feel the pouch swings a lot, you can add an extra zip tie to tight the pouch belt bottom with the backpack strap.

 Print Settings

Nothing spacial. work well with PLA/PETG
No support need it on the adaptor.
The phone need support on the gap of the cliping part.