This is a customizable Ultimaker Print Core holder. You can specify the number of slots horizontally, as well as the number of slots vertically. You can position the hook on the left side, the right side, the back side, or have no hook. Normally, no supports are required if you print with the back side down; however, if you position the hook on the back, supports are required for printing. Note that the dimensions of the hook are customizable, since the thickness of the Ultimaker side panels can vary (my printer has a thicker side panel on the right side than on the left side).
Print with the Back Down (no supports necessary), except if you position the hook on the back, in which case supports will be necessary.