Here's the layout for doing a pixscan mat photo box using a sheet of Dollar Tree foamcore (20 in x 30 in x 1 cm)  Dotted lines on legs are cut partially so they can fold. Reinforce with duct tape.

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The box lets you lay your cell phone on top and take a picture of a mat through the hole in the top.  I use a Pixel 3a, so if you use a different phone, check the height you need to be at to get the mat fully in frame.  This pattern is for 15" .  If you need it taller, you will likely need to use a second foamcore board to make it.     It is ugly, cheap and collapsible.   I'm sure you all could make it a lot prettier, if you are so inclined.   I suggest drawing around your phone once it is in place for a picture to make it simple to position the next time.