What are the simplest and most accurate ways of making one ?

I could draw round the mounting posts with the engine on a sheet of paper but that then would need translating to something that has the cross hairs to match up with those on the firewall.

In case it's relevant, the engine is a CRRC Pro GF45i which needs to be mounted inverted.

If I had the official mounting bolt pitches documented I could make one on the computer. But I don't [:@]

Thanks