FYI here is one that uses FN 90 mags it is top feed, but the part I think is nice is the bottom ejection through the empty mag well. I assume the OP knows about this since he is asking specifically for the standard mag feed.
some newcomers may not know so I thought I would post this since it is on my short list.
AR 57 Upper
There are possibly cheaper options out there if you look around.
