This is a modular frame system for the MMT (Modular Marker Tower) system.
The modular frame consists of a corner piece, two side pieces of different lengths, and a split corner piece.

With this solution, you can build as large of a rectangular frame as you need, without requiring a very large printer.
To connect the pieces, simply hold them together at the ends, and screw on any MMT-module on top (a cap is shown here in the last picture as demonstration).
You can also screw in any of the modular pieces underneath, like the different bases to elevate the frame off the surface.

You can also place this on multiple levels, to give more stiffness if building a really large modular tower.

There are several holes for D6x3 magnets, both on top and bottom, so that you can use it to secure the other 3DSM marker blocks to it, and as well secure the modular frame system to a ferrous surface.

Choice of filament material for the MMT:
This part was designed for use with Bambu Lab PLA Basic, but can work with other regular PLA of similar quality.
Matte PLA or Silk PLA should not be used due to their weaker mechanical properties compared to regular PLA.
 

Choice of filament color for the MMT:
Gray: Works well when scanning most objects in marker mode, and it will also work to assist tracking in geometry mode.
Black: Works well when scanning black parts, as it can otherwise risk overexposing the marker block, leading to loss of tracking.

The 3mf file for the different sizes has the following number of pieces per buildplate to be printed:

Frame Corner - has 4 pcs ready on the plate to be printed.
Frame Side - has 4 pcs ready on the plate to be printed.
Frame Side Split - has 4 pcs ready on the plate to be printed.

Frame Corner Split - has 4 pcs ready on the plate to be printed.

Frame Ends - has 2 pcs left and 2 pcs right ends ready on the plate to be printed.

Added a print profile for the A1 mini users, to be able to print the “Split” models that fit on the smaller printbed.

Added an introduction video for the MMT-system that you can find here: