Due to the design of the coupler an extension would need to be made from multiple pieces, like the magazine already is. It wouldn't end up being less expensive than buying a second mag.
Running the bottom mag empty (as an extension) will mimic standard 30rd mags - this is going to be the most immediate reliable solution if you want a full profile but don't want to run coupled mags.
We hear the request and desire for extensions, and do not want to discount them completely, however right now priority is placed on these mags and the next set of products to follow - which will consume all of our bandwidth. There are no immediate plans for a blank extension due to our initial focus being on the immediate scope at hand.