I'm looking (and not finding) a magwell for my Glock 21, Gen 4 that can be used with the large backstrap and would work with Taran Extentions. This combination of requirements has led me on a fruitless search. Perhaps my GoogleFu is not strong enough.
I have a Suarez for a Glock 41, but like you found out, it will not work with the backstraps or most mag extensions. I have only found magwells for the smaller framed guns that work with a backstrap. The Grip Force Adapters work with all Gens of Glocks, and then you can use whatever magwell you find that works for your mags with extensions.
Thanks for taking the time to respond. After about another thirty minutes of looking I just started emailing every magwell company I. could find and Carver Custom got back to me immediately. Their "big mouth" for the Glock 21/41 Gen 4 will work, I have to do a bit of work on the backstrap to use it with it, but it will work and most importantly, no problem using TTI mag extensions. I was so impressed with the immediate response on a Saturday morning from the owner, I placed an order directly with them and ... there you go.