That looks very interesting, are the grooves with space between them intentional?
Rather than making a square in photoshop, I’d recommend just an STL file that’s a rectangular box of the correct size. There’s also been discussion about making a work-surface facing routine, which applies to what you’re trying to do. In that thread @Darren mentioned doing it the “photoshop” way:
I don’t know if I did it the best way, but here was my workflow…
I made an 8" x 8" black square in photoshop. Opened it in meshcam, defined stock as being 8 x 8 x .375 (I think that’s the thickness, can’t recall), set maximum cut depth to .020, machining margin 0, and clicked on parallel pass only, 40ipm feed, 10ipm plunge.
I used the new .250 cutter / collet combo from Carbide3d, and took great care zeroing the board. Loaded the file, and it cut it without issue.
If you want to plan for “doubling-up” on current problem-solving and future utility, you could get the 1/4" collet and the 3-flute end-mill to do the job from the online store, and that’d speed things up substantially over using the 1/8" tool.