Finally good and cheap template - Gyprock or Plaster sheet.
Was thinking for a while how to make a template fast! And cheap. And more flexible.
Than an revelation.
Yesterday I've got a few damaged Gyprock sheets for free.
The plaster sheet is flexible enough to clamp it to the existing board and use Dremel plaster cutting bit. All done in less than 10 minutes without damaging the original board.
I've used Dremel routing attachment to have it vertical to the board and than sanded plaster slightly.
Will finish with water based brown tape ( find it in Jacksons shop), all around the plaster edge. This kind of tape is normally used for gluing a veneer. It's 30 years old tape from my days of making rocket models