I have a 7D and added this to the top of the Glidecam 4000 sled.
http://www.bhphotovideo.com/c/product/656408-REG/Glidecam_GLCWP_Camera_Weight_Plate.html
It's cheap and does the job.
I'm also trying to balance my 7D with the Glidecam 4000 Pro. I've gotten it pretty balanced using only 4 weights on the bottom sled, 2 on the front and 2 on the back, but the Glidecam tends to have issues rocking left and right when moving. Is this because the camera isn't heavy enough for it? Was this the issue you were experiencing and would the plate help this issue?