I took this as a challenge.
An only ways I could figure you could *possibly* do are:
Cut them all up in 2 up strips.
Place one on long way along the 10" side
Place remaining on short way parallel to 8" side
You can actually place total of 11 frames this way.
Cut two 3 up strips and place them along the long side
Cut the remaining on single frame and put them along the short side
Method 2 can easily fit all frames but I doubt your client would be pleased if you cut his negs in one frame each pieces.
Method 1 can just barely fit but rebate will be outside the paper. It will require you to very VERY carefully position each frame.
If I were you, I'd just get a box of paper and do two.... Method 1 really isn't practical. Method 2 is not likely please your client.
This is the best I can do to help you out.