Without pictures or any other details it's hard to visualize the situation. Which is probably why you got no responses yet.
I had that same exact problem following the dimensions of an online how to, I didn't check by making a mockup bag of the dimensions given and ended up with a ridiculously huge bag, that my gear filled only about 2/3 of the way up. And my stuff isn't sul, it's barely ul. 12, 13 pound base weight often. I did include compression cord system so the only part of the problem I had was that the bag was just too big.
What I realized was that even though I had made a compression cord system, it really didn't work when the bag was that empty. With all the other issues that first bag had, I decided to just salvage the hardware off it, use the straps etc, for future mockups, and call it a loss, or a gain, in terms of very valuable bag making experience in terms of thinking of capacity, making crude mockups to make sure my idea corresponds to reality, and so on.
Since the top closure is going to need to be shortened somehow if you reduce the panels, you're already looking to rip open the seams at least partially.
If you really don't want to redo it all the way, you can rip open each side seam to about 3/4 down, to above the side pockets if you have them, then resew it. Add in webbing loops for compression cord system at the same time. For a 24" high body, and a 12" width, each inch narrower, the long way, it is, 250 cubic inches are knocked off, or about 4 liters.
If you just want to do a quick and dirty hack, you can sew in the sides to one of the existing joints between back and sides, pulling them in about an inch or two, and stick in webbing loops or whatever, but that's going to be sort of ugly I think.
I started with 8"x12"x24 following the directions for a supposedly ultralight sized pack (no idea what the person who built that used for gear, those dimensions would easily have fit my old gear, non ultrallight) about, which is only worth doing it if you use a 3/8" or greater foam pad as an internal stiffener, folded up. That will use up about 3" of the internal volume the long way, which is about right re room I think.
I read some guys here note that it takes 4 or 5 bags to get it down, and given the small errors on the otherwise fine bag two I'm finishing up now, I'd say that's correct. For bag 3 I'm going to make a full mockup that is totally functional, just fewer bells and whistles, and lighter materials. Which will make the real bag 3 bag 4, exactly as noted re how many bags it takes to start getting it reasonably close to right.