make sure you are using a nice diaper FOE and not just any ol FOE. Some FOE is wonderful for clothing, but just awful for diapers. I tried and tried to make a FOE cover and ended up sad every time. Then, I bought a cover and the FOE seemed different. The wahm told me where she got her FOE, and it made ALL the difference. The plush kind you can get at places like diapershop.com (the 1 inch kind.. I don't care for any of the other kinds for diapers) is going to give you a nicer fit that won't cut into thighs like some of the shiney/narrow stuff can.
For getting a bigger leg hole, it's all about curve. Make sure you have a nice curve so the front of the cover and the back of the cover are wide and it narrows nicely right there through the stride. The less curve you have, the smaller the legs holes will be. I think a cover can be pretty darn narrow right there toward the front just where the thighs begin without comromising coverage. That's usually where I have the best luck adjusting the curve to get a bigger or smaller leg opening.
There are some good patterns on the market.. very baby and new conceptions both have patterns that would be good if the shape is just eluding you. Sometimes a pattern can really help. Adding to the rise can help, just be careful and don't get carried away.. 1/2 inch or so can make a lot of difference if you add it right there through the stride.