I'm a big believer in not sacrificing form for function. My goal is that, once you know something will work, make it look good.
To that end I decided that I wanted to taper the top of the rack to a rounded curve. This was very easy to do.
First, I decided on the height that the middle shelf would need to be at. Since my spools were 8" in diameter, it was easiest to just divide the sides in half, giving me 12" on either side of the shelf (minus 1/2" for the shelf's thickness).
I drew my line midway to represent where the shelf would go, then split the sides vertically, right down the middle. I bisected the top half of the side and drew a circle with my compass to represent the spot where the dowel would rest.
For the top point, I used the middle, vertical line and decided on a 2" diameter curve, easily achieved with a compass. I connected the edges of the curve to the ends of the middle shelf line, thus giving me a guide for my taper.
I wanted a slot so that you could slide the fully loaded dowel into place without having to remove the spools first.
I pretty much just eyeballed it. I used a French Curve to keep the slot the same shape throughout. Important note:
Make sure that, since the dowel is 1" in diameter, the end of the slot in the side is big enough to accommodate this. In my first sketch, I just drew the lines from the edges of the middle circle right to the outside and found that as it curves, the slot narrows significantly.
The pencil didn't show up too well in the photograph, so I've used Inkscape
to sketch the lines in.