So glad I came across your post cosworth34677 as I'm dealing with exactly the same situation and cannot find a low-cost solution. Lots of choices between $100-$300 bucks, but I'm looking for something under $30 bucks, if that!
I'm in the build process of my Bruder CAT Excavator conversion now, but having the same issue with the slew. My turntable comes apart after 3-4 rotations.
I noticed this post is over a month old, so please post a solution, if you found one. Otherwise, I will continue to watch this thread for possible work-arounds like the one I found here for 10 bucks...
I just purchased a BMX Gyro off Ebay per this build... maybe it will work?
http://www.scale4x4rc.org/forums/showthread.php?t=72245