When building with 20x40 V-Slot Linear Rail, and the project is completed, there is no way to add parts because the end of the rail is closed and a new nut can not be inserted.
With the drop in nut any place on the rail can be accessed. The nut fits the slot but when turned it locks in position.
At first I had my doubts about making this. It is so small and it may be too weak. I was wrong on both counts. With my Prusa MK2.5, I was able to print a PLA part that was very exact and strong.
I make two versions. One uses an M4 bolt that self treads into the nut. The other has an insert area for an M3 nut. Both seem equally strong and certainly strong enough for non structural applications.
This will only work with PLA, PETG does not print well. Also, you need to use '0.15 Optimal' setting on the 'PRINT SETTINGS' of Slic3r, perimeters is set to 7 and infill to 100%.