Love the tigers. Lots of good ideas for making your quilt longer.
I often have that problem and the very simple solution that I use is to make the top and bottom outside borders wider than the side outside borders. Inside borders are all the same size it's just the outside ones that take care of the problem. So if your side outside borders are 4" you could make the top and bottom borders 6 or 8" to make the quilt the length you want.
Please post photo when you finish the quilt, we all would love to see what you came up with. May in Jersey