You probably discovered that mounting any accessories on the printer'?s frame is a bit inconvenient. It is a good idea with these square nuts slid in the frame but unfortunately if you want to add something after the printer is fully assembled - it can be very ungrateful. Some of the areas are easy to be modified, like the vertical aluminium profiles - it requires just removing the top caps. However, some areas are very hard to access (like the Z carriage arm) - you would need to tear the printer apart to get into the groove in the aluminium profile. My first thought was to design a locking pin that would slide through the slot and then lock if rotated 90°. I have created rectangular holes on the sides of the clip to allow the pin slide through. Flat M3 bolt + washer or flat M4 bolt can be used. More details on my blog: http://ljwinkler.blogspot.ie/2014/01/frame-clip-and-locking-pin-for-velleman.html