The spacing is 17mm (assuming the popular 25mm motor breed). Can you drill the holes in some wood/something relatively thick, get it good, and use that from then on as a template? Can place the piece over the final part and use it to guide the drill bit. Just put a screw in each hole after drilling it before moving to the next to guarantee alignment.

Alternatively you could look at 1:1 scale printing a 2D drawing and using that as a template.

Might be worth picking up a centre punch and some measuring tools: cheapo digital calipers and a little metal ruler, possibly a scribe.