NECA may not have been able to give fans all of the Engie's buildings, but that doesn't mean you can't at least have some of them, thanks to these papercraft versions.
The sentry gun appears to be a good size, though the dispenser is only about half as big as it should be - and of course, if you want blue versions, you'll have to change the color of the files.
This is a good start, but a Level 3 Sentry and a teleporter would really go a long way in making things even better.
Actually in the site linked there is a link to blu templates including a level 3 sentry