Buddy Bucket Upholstery Installation (is this fitment normal?)

[badpeanut]

I'm finally redoing a set of '68 buddy bucket seats I bought from a board member (back in 2008--cost delayed this project). I installed new OER foam on both seats (upper and lower), and the upholstery is from Ricks Custom Upholstery. I've never done upholstery before.

The bottom upholstery (red arrows) is off the seat foam by 1/2"-3/4" in the middle. I tried tightening it from the back of the seat, letting it sit for a few days, and using a garment steamer on the vinyl portion of the seating surface while my butt holds it in place, but none of these have helped. These buddy buckets do not have a listing wire in this area like the '69-'72 buckets. I can't let the sun work its magic, because this is North Dakota in the winter so I'm stuck in the basement.

This fitment doesn't seem right. What am I doing wrong?