I use a cut down 20x24 print bag from Light Impressions to store my 8x20 negatives. I tried to cut downa 16x20 bag first but it was open on the 16 inch length. That left two sides open to easily remove the negative but in working with them, I found that they also had a bad habit of folding back leaving the negative exposed (uncovered) on one side.
I went over to the 20x24 bag because it was sealed on the 20 length but that was what I needed. I don't waste the top part of the bag. On some I cut it down to fit inside the now 20x9 bag to make an second pocket where I have placed some proof sheets.
The nice thing has been that since the bag is set up to hold a print, it is a little oversize on the 20 inch length. This makes it very easy to get the negatives in and out.