I found a better way to cut Sintra last night. I don't know WHY it didn't occur to me sooner. Maybe it's because I use my Dremel for 95% of all my projects, but I didn't even think of using the band saw attachment on our Shop Smith Mark V. DUH!
A band saw gives you a clean cut, unlike a Dremel which cuts and melts your cutting line. The band saw only melts the Sintra if you leave it in one place for a long time. The band saw lets you cut compound curves and angles MUCH more easily than the Dremel will. Of course, the Dremel was about $60, and the Shop Smith... well... we won't go there. :facepalm
Any band saw should give you really great results.