I found a great DIY pattern on Prudent Baby. You need a yard of fabric (I used quilting weight cotton), some corset boning and a couple d-rings. Each of these took me about 2 hours, but I'm slow. The second one definitely came together a lot faster, but I tend to stumble through things the first time I make them.
I used stash fabric that I bought earlier this year and I'm pretty happy with my fabric choices.
I added a terry cloth pocket on one corner of the cover. I'm pretty sure that "Udder Covers" have these. I'm not sure the pocket will be functional, but the terry cloth can be used to wipe the babies mouth after nursing, which is nice. It was a simple mod. I just cut the terry cloth to size in a triangle and pinned it into the corner, turning the edges under for a nice finish.
I chose to make a light coloured cover, as well as a dark coloured cover, because baby is coming in July and if I only had a dark cover, we might got hot nursing outside. Each of the covers probably cost me about $7 to make and I'm really happy with the final results.