This is awesome... We've got a nice little team going here, essentially testing drygloves. Different locations, different gear, different dive styles.
SHOWA gloves - what we're using here - are made right up the street from me... Right here, in the good ol' US of A.
Want gloves? Call them directly and ask for some samples. The phone number is 800-241-0323. Ask for two pair of whatever style you want, up to three styles. If they ask, just tell them you're with DEEP SOUTH DIVERS and are testing gloves for underwater use.
Cost = free. They don't even ask for postage.
Sekeli: Your idea sounds fantastic! You're essentially planning on making a ZipCuff... How cool is that? A zipped-in dryglove ring. Brilliant! You know you could sell a lot of those things, right?
Ean: The 660s and/or 620s indeed may be a little fatter and stubbier... But any difference is tiny. Call SHOWA and order two pair of 620s, 660s and 720s - in upsizes if you need to - and compare for yourself. If the 720s are narrower, the offset would be that they're stretchier, so they wouldn't feel tight while in use like the 660s and 620s would. Also - the 720s are naturally warmer than the 660s because nitrile is more insulating than PVC... So you may find that you don't need glove liners in the 720s like you do in the 660s and 620s. Lastly... FourthE's new Xerotherm glove liner is definitely an option. It's thinner and more svelte than the G2, and arguably just as warm... Especially under a 720. Personally, I believe that a glove liner is overkill with the 720, but if it's cold enough for you to use one, try the Xerotherm instead of the G2.
Glove liners are ALWAYS necessary in latex gloves, which is what the manufacturers have designed these rings to work with. Latex is thin and stretchy - which is great - but is fragile and holes easily... And provides zero warmth at all, which is why liners are popular. The nitrile 720s by comparison are warm and incredibly tough. You may not need the liners at all - and probably will not see a hole or leak ever unless something catastrophic happens to the gloves.