These are not exact, but close. 7" wide and 8" long of useable space.

Only reason I can't give you exact, is I have a plate I made to "hard install" a roll bag. After experimenting with plexiglass, I made the final one out of some 1/4 inch black plastic I got at work. Rounded the edges and bent an end lip of approximate 1 1/2" with a heat gun. Not sure what kind it is but it seems unbreakable. Made some flat brackets for under the OEM rack and then used the slots on the OEM to run bolts/washers down through the bag and secure under the rack. This way there is no damage to the rack if I ever want to take the bag off. I made the plastic plate, not counting the lip, approximately 11 1/4 " wide and 9" long so the roll bag fits snugly down in the lip.

I would post a pic, but it looks like way more than I want to get into to do so. If you want me to email ya one, I would be glad to do so.

Also, I have a extra plate of the same size when I was making mine. Has one small scratch on it, that is hard to notice on the lip. If it would work for your project and you pay the shipping and wrapping, you can    have it for nothing

How do you have that rack attached?