I need a pocket!

[undefined]

That is a charming back story.
I have a lightly used old style Dive Rite Vertical Zip Pocket--it measures 9X6X2 so it should fit your 5"x8.5"x2" object snugly. It's yours if you want it, and you can reimburse me the shipping costs after you get it (should be around $10 USPS priority). PM me if you are interested.View attachment 894640

That's so kind of you to offer. I'll send you a DM.

 

[Endler's]

Halcyon Weighted Bellows Pocket is arguably best choice, & @ 6.5 X 9.5 X 2 inches, will fit your device.

 

[randini]

butt pouch like sidemount divers use. just clip it off to the base of your plate with a pair of double enders once your tank is on your back.

Or even one of those thigh mounted pockets, but mounted on your wife's tank.