I needed a 45° cutter to make miters in some 5mm foam board without cutting right through. It was a 30 minute design and a 40 minute print so it's a bit rough and ready but does the job that I needed it to. Pics of the thing in use and also the flash diffuser I was making are shown above.

All you need to do is draw a line on the centre of the joint you want to make (on the inside of the fold, obviously). Position your rule/straight edge along the line and carefully draw the blade through the board whilst keeping the front edge of the cutter against the rule. Do the same on the opposite side of the line and you'll have a small wedge of foam to discard and a neat 90° rebate allowing you to fold your board.

It takes a standard Stanley/Box Cutter blade, the type that are 62mm across the sharp edge. If a different size blade is used it may not work as intended as the amount of blade sticking out beneath the base might be different. I just hold the blade into the rebate while using although I suppose you could tape it in place if you prefer.

PLEASE NOTE: The 5mm version in the example photos is the only one I've tried. The other two (for 3mm & 10mm foam board) are the same principle and should work. I have no need for these thicknesses but a comment on my Reddit post asked about me making a set of them. If you try them and they work (or even if they don't) please leave a comment.

ALSO: Obviously these type of blades are ridiculously sharp and care should be taken at all times whilst using them. I can't accept any responsibility if you use these cutters and chop something off that you shouldn't have.