I don't have a large printer so I split the horizontal pieces from Michael Klements' Lab Rax 10" server rack system (seen here) with dovetails so they'd be easier to print. I've included the two-part solid piece and the three-part vented piece. These can be a pretty tight fit if your print settings are good. I like to snap the halves together, then lay them on a hard surface, tap them on the seam with a rubber mallet (to create a flush connection), then use a soldering iron to weld the parts together.