I have never been asked to remove my regs for inspection and I know they see rougher treatment with day to day diving so I just wrap my regs in my wetsuit and put it on top of my BCD in my carry on. No special care or bag needed.
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Google Amazon for packing cubes. I've not found any need for padded regulator bags. They add a ton of bulk and weight for no good reason.
I don't use a regulator bag to protect my regs from stuff, I use it to protect my stuff from the regs! A chunk of solid machined brass doesn't take much to break something given a couple inches to move and some sudden acceleration.
Back when I used to have to reconfigure the same sets of regs when going between singles, doubles and sidemount (now I have more gear), I used to have to move hoses around a lot. In addition to taking some cell phone snaps, get some port plugs of contrasting finish. If you remove hoses from say a black finished first stage, use chromed port plugs to temporarily fill the ports. When you arrive and reassemble you'll spend less time puzzling over which ports you had the hoses in. Particularly helpful if you have multiple first stages that are configured differently.
I understand why you do this, but I think It would be easier for the OP to just put the second stage in a heavy Ziploc and then wrap something around the first stage to protect it and other items in the case. He should know the inflator and SPG come out of one side and the regs come out of the other.
Wait, who did just do that? What is the new size?...Unfortunately they just reduced the carry-on size by two inches...
Intrigued, ... should I read this as in you wear your BP with attached dry bag (with stuff in it) like a carryon backpack?... Normally I pack all of my tropical gear into a dry bag and attack that to my backplate and wear it, should have done it on that trip too.
I did a post with some pictures a while back using a VDH flat plate and it works as well with my Freedom Plate.Intrigued, ... should I read this as in you wear your BP with attached dry bag (with stuff in it) like a carryon backpack?
If so, what do you find are the advantages over putting the BP & stuff into a carry-on backpack? Weight?