This is 1:1 scale template. DO NOT use below image. Open up the PDF, print, cut and use it.
http://findjdm.com/hud.pdf
Here's the detail.
I spent a long time to come up with this template, so I can say it's VERY accurate. X-Acto knife will work great. You can use Dremel, but it'll make a mess under the dash. It doesn't have to be perfect since edge will be covered by HUD trim cover. Good luck!!
Disclaimer: It's not my fault if you mess up your dash.