I make my own stencils using transparency plastic (you remember, the overheads teachers used to use?). Your double bladed exacto knife puts you one up on me (where did you find it?). you just have to be careful that you never remove the plastic across and intersection. Once I figured that out, stencils are pretty easy to make. Then I mark it with pounce chalk.
Sizing is more difficult - but a plastic stencil at least lets you lay it on top and see where you need to adjust.