I have the luxury of a "cutting laser" to cut routing templates, however, they could probably made with a scrollsaw/jigsaw (carefully).
My Hummer build should detail how I do it... though Master  Yoda  Dutchy could provide better insight... though he now has a laser as well (sorry for the reveal Dutchman).

If your first cut is too short... Take the second cut from the longer end... LBD