Well more or less remix from the 45 Scribing Jigs by Taifurious. I made it straight and shorten it a bit. Size from 1mm to 8mm line. Works really well with those box shaped Gundam. Symmetry lines between left and right.

How to use this to get 45?

1. You make a grid lines using 2mm, 4mm, 6mm or 1mm, 3mm, 5mm (see picture)
2. Cross those box and voila you got yourself a 45 (see picture)

1mm and 2mm might be hard to hold steady, I suggest use a sticky tack to hold the part and the jig to make that line

Use this to DRAW lines. Use dymo or scribing tapes to scribe. I don't recommend to use the jig as a scribing tape. It will dull your excalibur or worse, broke the tip off.