If anyone is interested, more information on my customized D-bot can be found on my website: Wildbot3d
I added slots to the motor mounts to allow you to slide the stepper motors back to tension the belts or forward to loosen them. I always thought the absence of belt tensioners was a major oversight with the D-bot. I've seen (and used) several designs/methods for tightening the belts but I think this is a simpler solution.
Partial credit has to go to Mark Rehorst as that's how he tensions the belts on his UMMD.