Do you require a lot of retention when your pouches are open? (ie, are you doing anything dynamic when looking at your GPS/switching radio chanels?).
Radio, rarely. GPS, absolutely. Being able to nav in dense forests without having to get the GPS out of a pocket constantly was one of the primary justifications to work on that platform.
If not, I could see two retention alternatives: Velcro/3M Dual lock backing on both the pouches liner and the back of your devices, and/or a couple of velcro "one wrap" horizontal tabs in lieu of your bungee. They won't secure your devices as tightly, but that might be enough for what you'll be doing.
I did add velcro the entire width/length of the flip-out panel and tried it, but I wasn't satisfied with the retention (although in retrospect the Velcro I got at the local sewing place isn't great quality, it might be worth trying again with better Velcro). I do however think it's a decent option for secondary retention.
The other option I can see is the clear vinyl sheath, then again it will only provide limited adjustment if you intend to change your devices over time. (although you might also make some sort of adjustable vinyl "wrap" with velcro sewn on the sides that would mate with a velcro liner on the pouch itself).
A friend has the EMDOM pouch for the Garmin and has had issues with fogging/condensation between the screen and vinyl. Generally the backlight is strong enough in the shade that the condensation doesn't make it too hard to see, but when it's bright it makes the already hard to read screen even worse.
An adjustable wrap that is velcro'd to the liner but hard sewn on one/some sides to make it more secure could be an idea worth exploring.
On a side note, it's hard to figure out your final design on the GPS retention system. However, if you kept the original sleeve design shown before (sliding the device through the bottom portion), why don't you just make your retention cords adjustable (the red ones that hold the pouch at that 45-60° angle) so that the pouch can be completely flipped open (180° angle) when needed. That would make for a quicker way to remove the GPS unit from it's sleeve and insert it back. All you would need is a bigger length of cord and one or two cord locks.
My bad on that, I do need to take better pictures
The two cords that hold the pouch open go around and attach to the buckle's webbing; they are the ones that also keep it closed tight. This allows one-handed closure.
Doing what you're suggesting would indeed make it much easier to insert a device from the bottom. I'd need to change the way the cords are setup as right now I can only have ~2 inches of slack without them getting in the way, and that isn't enough to make it lay flat. Worth thinking about too, thank you