With our colors and Stainless Steel caps we have 33 different parts, there is just no way we can do different top and bottom caps.
Also as mentioned we really like the slim profile, to add a slot the cap would have to be considerably thicker.
Then to top it off you have to have airflow, sure it would work great for some devices where there is a built in slot but others mounting flush would cut off the flow. You can always use non flanged cartomizers and nudge the tank down.