MYOG sleeping bag to insulated pants conversion.

That looks good. Clever to get your mom to do the sewing : )

I also originally started with a pair of jeans.

So, what happens when you bend over? Is there enough room for your rear end?

I didn't leave quite enough material in the back, so it does give me some plumbers crack, but the pants stay on, and I'll most likely have my wool bottoms on too, and hiking pants on the outside.

That is a great idea!

How would you have made them differently regarding the crack issue? :) Would you have just cut the rear "taller" or added a gusset in the crotch, or what?

The first pair of pants I did were tight around rear. So I moved "the point" out and the rear up and they fit better – about 1/3rd of the way down in http://www.backpackinglight.com/cgi-bin/backpackinglight/supplex_pants.html

After I try it out in the field in a couple weeks, I might add an inch or two of extra fabric around the waist and elastic since I have a lot of fabric left over.