One thing you could try is shrink film… the type you apply on a window for winter, then hit it with a hair dryer to shink it.
Create a ‘tube’ of it with one of those food sealers to melt the plastic together, then seal up one end of the ‘tube’, slip the phone in, then seal up the other end of the ‘tube’. Then hit it with a hair dryer’s air blast to shrink the plastic.
Try to position the ‘tube’ such that the joined ends which made the ‘tube’ are off to one side of the back of the phone, so the camera’s not blocked.
That should give you a water-tight covering that’ll keep the shot out of your phone, and it’ll fit inside the regular case. The plastic film is thin enough that it shouldn’t interfere much with sound transmission as regards speakers and microphone, nor with the touch screen.
As to the USB port, there are little rubber plugs you can get that’ll fit in there to block water (and shot) from getting in there. Remove the rubber plug to charge, replace it to use the phone (or install a wireless charger inside the phone so you can permanently seal the USB port… this is made easier because this phone can be taken apart and the parts replaced with new… you can solder the wireless charger leads internal to the USB port, then just never break the plastic film over the USB port).