Looking at photos of the Trail Pro it does seem to have a slight taper. Still, it should be really easy to produce a pattern. Since you have the sleeping pad I would suggest just drawing round it in chalk - add a bit for the thickness of the mat and another 10mm or so for the seam.
You have a couple of options for the opening you insert the pad through, maybe an extra 25mm on the bottom edge for velcro or a pocket similar to those found on pillow cases.
Either way, 10 minutes and a couple of sketches should be all that's needed to work it out.
Evolution stinks, I want my tree back