Tenor Sax Gig Bag
The mandolin gig bag is the wrong shape, namely not tall enough iin the midsection for my scoped PS90 TR. It also has too much slack at the bottom end. It would have been stealthy though, because the profile of the bag says musical instrument, not gun.
I took CD's suggestion and checked out saxophone gig bags. The Alto sax bag was too short. The Tenor sax bag fit perfectly and has the mid length bump that gives the scope plenty of clearance. The model I bought from Sam Ash is the Jean Baptiste Tenor Gig Bag which lists for $119 but was on sale and in stock locally for $59. It is lined and padded and will give your PS90 more protection than an unpadded tennis racket bag.
The front pocket easily holds 3 PS90 mags flat or 4 PS90 mags stacked with room for three 50rd boxes of 5.7 ammo. You could stuff even more mags in the main compartment with the PS90, so you can haul lots and lots of rounds with this set up. The back side has backpack carrying straps, so I could take it to the range on my motorcycle or hike it out in the country. The fit is very good and the PS90 slides in without any binding. It also fits my GSG-5 22LR (with optional retractable stock and the mag removed). The down side of the sax gig bag is that it has the profile of an assault rifle more than an instrument.