Backdrop stand extension arm

An extension arm for a third backdrop stand, designed to support longer horizontal bars in the middle while distancing the stand from the backdrop.

It comes in a few variants to accommodate popular bolt threads as follows:
- longer 1/4" thread: l = 40 mm, d = 7 mm
- longer 3/8" thread: l = 40 mm, d = 10 mm
- shorter 1/4" thread: l = 10 mm, d = 7 mm
- shorter 3/8" thread: l = 10 mm, d = 10 mm

The remaining dimensions are the same for all models and shown in the 2D image.

The arm should be printed on its side with snug supports; this placement also maximizes model strength. The mount should be printed upside-down with no supports and with bridging at the bottom of the slot. I recommend four walls and gyroid infill for added stiffness. Once printed, slide the arm into the slot in the mount.