Unwilling to have a custom frame made for them, I headed down to Ikea where I found 520 x 520mm RIBBA frames (Article Number: 601.798.01) had a picture width of 490mm, just enough room for two ATX motherboards side by side, or 4 mATX.
Because the motherboard was thicker than the maximum 30mm depth, I had to remove the IO ports to make them fit. Unfortunately, I had some trouble melting the lead-free solder with my project soldering iron and I had to pry them off with pliers. Many scratches and pricks later, the thinned-down motherboards were hot glued onto the frame. A few screw holes for fastening were also added for good measure.
As the frame was now much heavier than Ikea designed it to be, I was afraid the original clips would not hold. So holes were added to thread another wire for hanging. Hope this inspires you to give new life to those old / non-functional PC components!