Y'all seen the Haley Strategic Flat Pack Plus?
https://haleystrategic.com/shop/soft...flat-pack-plus
In grey it doesn't have the molle or velcro on the exterior. This looks like a very solid option for PDW carrying that can also be expanded to haul travel gear if you want. The option to integrate a chest rig into it on the fly moves towards the extreme end of LARPing, but if you're using a PDW as a potential primary weapon and need a chest rig, this could be a nice option overall.
Edit: Ah, I just looked at the dimensions 14" tall...means it won't fit a Rattler straight up and down, at an angle though shouldn't be a problem (particular with a 20-rounder in place).