Can you fit that in your puzzle?? Cause i have no idea what puzzle you're doing. I just thought it would be nice to help. But i do not have any puzzle as reference.
Anyway i hope that was right.
I'm confused....I thought rectangles had 4 right angles and therefore they didn't have sloping sides, but.........going through possible 13 letter words, if parallelogram doesn't fit, quadrilateral is another possibility.