Do you have a solid or tone-on-tone in your stash that would work with these blocks? If so, you could break up the quilt itself and not have to worry about a wide border. For instance, you could put together a small piece 4 blocks wide by 6 blocks long and then add a 4" border of the solid or tone-on-tone to each side. That would give you a quilt top of 24"x32".
Add more of your hourglass blocks to each side (maybe 4 more rows to each side and 6 more rows to the top and bottom to keep it symmetrical) and then add another 4" border to each side. Now your top is 64"x88" if I've figured it correctly, and I think you've used 256 hourglass blocks.
If you want a top that is bigger than that, you could add another border somewhere else in the middle of the quilt, or you could use the remaining hourglass blocks to create a pieced border of some kind to go around the outside of the quilt (I haven't figured that part out yet!).