Sorry it took so long to get back to you.... Out on the water for about 9 hours today
THe hatch I installed is the Hobie Vertical Opening Hatch. Here are the measurements I took:
Outside Dimesions: 14-7/8" x 10-3/4" (I'd say round it to 15"x11" for your cardboard cutout)
Inside Opening Dimensions: 8-1/8" x 12" (There's a little lip that the hatch lid rests on when it's closed, so I measured it there since that's the narrowest the opening gets)
(In the pics the measurements look off because of the angle of the camera)
I'd also say to make sure there is nothing structural in that section of the kayak before you cut into it. My Passport has two structural foam blocks on either side of the hatch meant to support weight if someone is standing up there. But the hatch fits between them just fine and I didn't have to alter them in any way.
Hope this helps!