Ah! Now I understand your question. Well, the answer for me was 2.1-2.3" tube, whatever that means. Basically you want something just barely under the OD of your housing. The 29" tube was all I could find at my local WalMart that met the need. My bike shop had hi-quality stuff, I'm sure, but I was too cheap to buy theirs and then cut it up into loops. The WalMart tube had the advantage (for us, if not for biking) of being relatively thin, and thus flexible.
I would be a little concerned with the Mk2, because the key piece of this is the room for the roof to flex. The Mk10 and the Atomic both have a deep groove where the SPEC boot fit. The stiff side walls of the boot formed the "room", and the flexible "roof" of the original SPEC boot (or the inner tube in our case) let this outer "chamber" move with the piston. Total volume change is 0.25-0.5cc when you first pressurize the regulator, and less than 0.1cc with breathing. If you have bubbles in your grease, of course, then the boot will need to have enough volume to supply more grease to make up for the ~75% loss in bubble volume inside the chamber at 99 feet. I don't see a groove on my newest Mk2 housing that overlies the environmental holes,
View attachment 159390
so I don't think there's any volume that can be created with an inner tube.
As we saw with the tight inner tube on the Mk10 on the pony, the inner tube doesn't flex much at all, and so won't transmit the increasing sea water pressure.
Earlier in the thread, we had a brief discussion about whether a reservoir is necessary. To recap: if the reg is
perfectly filled with grease without bubbles, then the inner tube over the holes will transmit sea water pressure to the piston, and probably give enough to be safe, BUT ON THE FIRST DIVE ONLY. The reason is that when you pressurize, some grease squirts out of a tight closure as the piston rises and seats. There's not much further movement during the dive, so the inner tube MIGHT be adequate.
But when you depressurize, that volume has to be replaced. Since the tight inner tube can't go deep enough into the environmental holes, then air is going to seep in over the day/week/month between dives. Next dive you have bubbles inside the chamber, and you see what I found when I dived with the pony - IP drops like a rock. I don't think you can do it on a Mk2 or any reg that doesn't have a place for the reservoir of grease under the inner tube to sit. Sorry!
Re: the IP gauge, you're right! The DiveGearExpress one is nice and compact, and inexpensive - but also moderately sealed inside its cover. Seemed too nice to get wet inside - I have one too!
My (more expensive) homebrew used a Chinese P.O.S. $8 gauge whose case was so loose that water flooded it easily, allowing it to to measure differential IP at depth (which should stay the same as your 1st stage compensates). After the dive, I just pulled the cover off and rinsed and blow dried it before it rusted.
