Though I have not made this one yet, the 'rules' should be the same. Yes, you can make a template of any ruler, no you add the length on the 'wide' end (the narrow end would result in a point very quickly).
BUT, I would just take my longer ruler (I have a 5 X 24 one) and lay it right next to the '10 degree ruler', move the '10 degree' and cut what needed to be cut. If you can only cut with one hand make sure that your first cut in on that side. ie: right handed... first cut on right side, that way when you lay the longer ruler next to the 10 degree ruler you will be cutting on the correct side of the ruler.
I hope this helps.
Good luck and take pictures!