The covers are designed for the long handguard (16.75") for both Keymod and MLOK and sit tightly in the rails. A rubber hammer is recommended for assembly.

For the design I took measurements and used the official technical drawing of the rails.

Unfortunately the rails come loose due to muzzle pressure, which is why I recommend them more for airsoft, although they can work in live weapons, depending on the muzzle adapter.

For other handle lengths, cut the printed part to size.

print settings:

nozzle: 0.4

layer height: 0.2

material: doesn't matter, i tested PETG and PP

raft: no

support: no

orientation: external surface on the print bed

post-processing: 

clean the part of possible stringing and insert it witch the rubber hammer.

NOTE: I assume no liability for improper use, use at your own risk. Be sure you know exactly what you are doing. Improper use can result in damage or injury. To be sure, ask your gunsmith before installation.

It is in my country a permission free gun accessories, but if it is prohibited in your country, do not download or print it. 

Note your local laws.