You could have multiple sentrys, like 4 (maybe in 2 lines of 2) with 2 "inplace" read to go and 2 underneath the first 2 on a time delay elevator / platform that rises and goes down.
Basically, have the first 2 on platform that has a delay before it lowers down and brings up the new sentrys. The delay would be obviously the average time that the sentry gun would run out of ammo before needing to be replaced. After that, the first 2 would drop down and be hidden, and the otehr 2 would rise and take their place.
I think there is a brush you can use to block the sentrys line of sight, and combne that with block bullets and would wouldnt need a wall to divide the 2 (maybe you could use a fence / mesh instead with the line of sight / bullet brush).
Only way around it that i can think of. Plus, it would look cool ingame in terms of it all happening rather than just having a permanent sentry