It's hard to suggest borders without a photo of the quilt so far. To add that much size (20"), I personally would add another row of blocks all the way around, then a 5" piano key. There are no hard and fast rules for a piano key border, but I would say about half the size of the block is a usually the max size you would want to go, so in your case, around 7 or 8 inches. The piano key is normally the last border on the quilt. So you have about 12 inches to fill. You can do a series of smaller pieced borders for a medallion quilt look, or you can find a 12" block you like. I think 12" of plain fabric borders might be a bit too much, but maybe with interesting prints it might be OK. Need to audition it. If you do go with a pieced inner border, you can size your keys so the seams line up with the edges of the border blocks, or put a narrow stop border then your keys. One thing for sure is to put a row of stay stitching around 1/8" from the outer edge of your piano key border so it doesn't pull apart when being handled while quilting.