I have a MP5K clone (MKE AP5-P) that has a foregrip from A3Tactical, and the flat mounting base is 25mm wide, and the Midwest Industries forearm is 17mm wide. This intrudes on the use of the adjacent rails at the 45 degree.
I found the STL files created by James Stubbins here: https://grabcad.com/library/mlok-compatible-plug-spacer-1
I printed (2) in Prusament PETG at .3 layer with 15% infill to test fit. Then I remixed his 5mm spacer to span the 2 openings on the 45 degree interface, and handle the bottom of the Unity Hotbutton switch which will fit into 1 of the 32x8 MLOK interfaces.
I use painted supports with triangles and don't fully cover the surface so it is easier to remove. That part is hidden by the mount anyway.
You'll need 2x screws that are 5mm longer than the screws that come with the Hot Button. 10-24 x ¾" flat head screws match the MLOK t-nuts that comes with the switch and are a perfect fit. You can get black-oxide alloy-steel ¾" screws in 10-24 from McMaster-Carr with thread-locker already applied for about $9 for 25ct.
Version 1.2 Uploaded:
This has an inside radius on the short-end that bridges from the full-length MLOK slot. This is for people who can machine it from the STL, and thus no corners to try and cut with a round tool. Plus it looks better.
The author marked this model as their own original creation.