For the record, and in my experience, I wouldn't think twice about modifying a standard amazon seat cover set to remove the top portion.
If you've ever installed a set of seatcovers on a jeep, many of them come with straps that you stick through the back/bottom/rear of the seat, and wrap around and clip underneath. So, if you removed the top part of the seat cover (aka cut off the top part, but leave the straps and the bottom portion), then it should still very securely attach to the seat.
Of course, if you're looking at pricier options, you'll want to make sure to check the installation instructions (or installation videos on youtube) to make sure the seat covers are designed in this fashion, but I believe most of them are. Hope that helps!
Then again, if you're looking at pricier options already, maybe you just have bottom portions custom-made, or a set made with the 392 stitching!