For the cardboard trick just slide it into the drum and put the lid back on. No tape needed.
As for the load outs you posted, off the top of my head get rid of gloves dangling. Put them on your hand or put them in a pocket/ ruck. Otherwise you will end up losing them. Ditch the idea of carrying mags with your drums as well. You have a platoon of mates carrying mags. If shit gets that bad that you have to resort to mags there will be some available.
Another belt type suggestion for you is the velocity OUB. Its shorter top to bottom and allows you to hook it to your pants if desired. Personally I would not run a belt with blast protection in it. Not enough juice for the squeeze.
Optics on the feed tray are ok in my opinion. Keep in mind it's an area suppression weapon. Use a red dot on it. It will be accurate enough for minute of man if you have to clear rooms and if its an extended range engagement put the dot on them and let the beaten zone and come of fire do the work. As for the forward grip, if they can mount aiming lasers on the sides of the frame, then they can mount a grip under it.
I may have some more later, have to read back through a bit.