Drafting a button front fly

I'm making the fiance a pair of flannel-backed satin* jammies so that hopefully he can roll over in bed and slide under the sheets instead of dragging them with him. Being that they are going to be satin jammies, he wanted to make sure they don't look like girl's clothes**, so he wants a fly. A fake fly would be fine, because it's just for looks and not function, but I think a real fly will actually be easier than a fake one.

* I have to admit, this fabric is a point for Joann. As much as I complain about their lack of interesting fabric, this stuff is pretty cool and I couldn't find it anywhere else online.

** Why is it that it's totally kosher for women to wear men's clothes, but not vice-versa? I guess this is one place where we have the better side of enforced gender roles, for what that's worth.

Since the pattern I'm using doesn't have a fly (or pockets, for that matter, but those are easy), I'm trying to draft one from a pair of jammies that he already has. After extensive measuring, here's what I came up with:

Gotta be careful to remember that the pattern as is has seam allowances, and to add seam/fold allowances! So all seemed well until I tried to trace my new pattern onto the old pattern:

Uh oh. The crotch seam curves before the end of my fly. Some of that is ok because I can just straighten the top of the seam, and it'll get taken in enough with the elastic waist. But the rest of the curve at the bottom... I'm not entirely sure what to do. The sample jammies are straight all along the fly, easy to tell because of the plaid. So I think I'm going to basically assume I can make the fly straight and even out the crotch seam somehow to fix it. Flying by the seat of my pants here, which is only appropriate.