Magnetic building board
thingiverse
You can get the magnets for the triangles on Amazon: https://www.amazon.ca/gp/product/B00IYFO79I/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1 And for free building: https://www.amazon.ca/gp/product/B00IYFO7KM/ref=oh_aui_detailpage_o01_s00?ie=UTF8&psc=1 Use 1-1/4" #10-32 machine screws to bolt on the magnets to the triangles and use an 18 gauge galvanized steel sheet (mines 24x60") for the building board. Take a file and make the vertical edge as square to the building surface as you can. Shouldn't have to change it that much.
With this file you will be able to print Magnetic building board with your 3D printer. Click on the button and save the file on your computer to work, edit or customize your design. You can also find more 3D designs for printers on Magnetic building board.