I actually just bought this piece on a whim, as I wanted more space, but don't have rack mounts. It is actually way more useful than I expected.
https://www.jandd.com/detail.asp?PRODUCT_ID=FMW3-2NDS
Came in perfect condition for the record....it's not blemished. And I spoke to them on the phone...their warranty states they don't warranty closeouts, or blemed items. The dude on the phone laughed and was like "if it's got our name on it, we don't care how you got it...we'll take care of it".
It's always on my road bike now, but can easily be put on my mountain bike.
I wouldn't trust the bungee cords if you plan on mountain bike packing or anything, but if you're just use it for commuting or road touring, it can fit quite a bit, and is extremely solid and stable. I may try to use some solid straps rather than the bungees for some heavier duty use.