For 8x10 and 11x14, this is easy. I get the standard sized 3 mil polypropylene shield and place them inside archival paper envelopes from Light Impressions. For 7x17, it's a little trickier, and I'm still in the middle of a solution. Hopefully, those at the far end of good solution will enter this thread and help us all out. But try this: buy the 16x20 shields and cut them in the Rotatrim on the long axis. Voila! 8x20. At the site below, the company states that customs sizes are available (for a highly customized price, I'm sure).
Now if I just had an archival paper envelope to place those sleeves into.
Try Impact Images. http://www.clearbags.com/
They have a large number of sleeve sizes that should fit any size negative. They are also excellent print sleeves. Very clear.
Contact them and they will send you a sample and a size / price list.
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.