To repeat a theme: this project focuses on the CNC mechanics, and the shortest path to building your own and making it work includes not following me down the fancy box rabbit hole. Yet. This is enough to get started:

If you have access to a table saw, or some other way to make straight square cuts, you can make a slightly less haphazard version of the same thing:

While square parts look good, that's still only "slightly less haphazard" because the four frame parts are sized by eye with no measured dimension. Keeping it simple!

Placement of Z over X&Y

The frame's job is to hold the vertical axis over the horizontal axes. That's the relationship that matters. Here are some dimensions to help set up that relative relationship. The numbers are millimeters, but "looks like this" is close enough.

Left and right clearances

Just a little bit less simple

Closing off the lower back of the frame will go a long way to making cleanup easier.

A bit of cardboard and tape will do the job: 

For just a little more effort when cutting parts, a couple more pieces cut to the same width as the vertical transverse piece will both close off the back and also make the frame significantly more stiff. And if you're going that far, a third piece across the top will add another increment of stiffness to the Z axis.

Chunky material like the 3/4" (18mm) slabs illustrated here will probably be plenty stiff enough on their own for a minimal four-piece frame. If your material is thin or otherwise uninspiring, the three extra pieces shown above can very significantly stiffen the frame. If using the narrow horizontal pieces behind the top and bottom edges of the vertical Z axis support, they should be attached along those spanwise edges as well as at their ends.