Here's some information of the pcb board holder and a parts list to use.
The 2020 aluminium profiles are available to order from many places and manufactures, I bought these from aliexpress in pre cut lengths and M6 screws can be used on them, many of these only handles up to M5 but with M6 the small washer between the large ones on the clamp has the same thickness as the board, 1.5mm.
I assume similar profiles are available in the US as well, ¾” is about same size if you wish to use imperial instead and the M6 is about same size as a ¼” screw (won't fit in the 2020 profile though).
Since washers are punched there's one edge that's slightly sharper, I don't know if it matters but I turned that edge towards the pcb board with both of the large washers, hopefully for better grip.
The corner brackets has small guidings for the profile, on the adjustable profile you have to break it off to be flat against the surface.
These parts is what I bought, there's a lot of other stuff, sizes and other manufacturers to use instead if you don't find the listed parts, and some extra washers and screws won't hurt.
Adjust profile length for your need, these length works well with pinball pcb boards.
Parts List Aliexpress:
2020 profile 500mm, 2 pieces for the frame
2020 profile 300mm, 2 pieces for the frame
2020 profile 350mm (minimum 300+20+20), 2 pieces for adjustable profiles
2028 bracket corner fitting, 8 pieces
20 T Spring nut M6, 32 pieces minimum, buy a pack of 50 if you wish to add more clamps or attach other things like holders for components etc.
Star Knob M6 x 35 mm male thread, 4 pieces for adjustable profiles
Handles UXCELL 20 x 110, 2 pieces
Knurled Knob M6 x 10mm, 4 pieces minimum but 6-12 is better for clamping, 3 on each side for large boards
Small M6 washer 1.5mm thick 12mm Od, 1 piece/clamp + 24 minimum
Large M6 washer 28mm Od, 2 pieces/clamp
M6 x 10mm screw, 16 pieces minimum
Red rubber feet M6 x 55mm with nut, 4 pieces ,this is a destaco spindle used on their hand clamps
The Black Rubber feet has 8mm long M5 thread + 10 rubber, 4 pieces
I had these at home and some M5 T Nuts but finding something with M6 x 6mm instead shouldn't be too hard. Depth for screws in profile is 6mm but use washers or cut the screw if it's too long.
Distance 25mm Id 6mm Od 12mm, 4 pieces for Star Knob, you'll probably find something suitable if searching around.
Good luck and hope this wasn't too messy
Edit:The small M6 washer can be used with M5 screws and profiles as well