I have a binder that works well enough. It bugs me that it only lessens the size of my chest instead of making it flat, but I know that making a D-size chest flat might not be possible. The problem I have with it now is that it's short and I get a painful pressure around the bottom of it after a few hours. Also, depending on what shirts I wear, there is a visible line where the bottom of the binder is under my shirt.
So I am looking for a long binder. I don't have much money, so I need to amek sure that whatever I get will work. The problem is most reviews I can find are along the lines of "I am a B cup and it works really well." I can't find any reviews saying how they fit bigger people. What would recommend for someone with a D-size chest?
Also any advice on what to do for the shorter one would be a big help. If I took out the bottom seam would that work?

I use the underworks 997. I have a bigger chest (I used to be a 36F). It won't make your chest completely flat but it does help flatten, and I feel it filled in the gap between my chest, and where my chest ended. My experiences with the underworks tri-top was pretty negative. My chest wasn't "firm" so if I went "down and out" my chest would sort of stick out of the bottom and I never really felt secure. It also rolled in the back in a weird place (plus there was a gap in the front between my skin and the binder.

There are some other things you can do to help conceal a chest unrelated to binding. Wearing patterns, layering, and some other things that I can't think of right now do help. Also, if it's possible increasing muscle mass in your arms and shoulders will make your chest appear smaller.

+1 to Nygeel.  I wore a 997 from Underworks as well.  I think I was a D or DD when I first started binding.  My tissues were already kind of saggy and definitely broke down more from years of binding and then T.  Put them straight down for years until I was able to "down and out". 

I wasn't totally flat in the beginning, but over time I became that way.  Even had people handle my chest while stealth without suspicion AFAIK.  Even in the beginning though, it was a really good binder.  Never bought another kind or brand.  Probably owned 7 or more over the years.

Thank you very much for the advice!

I think the 997 is the only full length binder (with the exception of others on underworks that are like wearing a wrestling singlet).

I'm a DDD and I wear a tri-top. It doesn't get me flat, but it helps and I do pass. I'm just waiting for T to help with tissue.