Wash it out, put some antibiotics on it, and then wrap it up - I guess the only way to go is all the way around her belly. I know you can buy OTC wound sprays to clean it out (a lot of them you might want to dilute depending on their concentration), as far as ointment, I've always used Corona - but I've never had a wound that deep or big, so I don't know if it would be the most effective; also, it doesn't come it spray form, but I find that my horses don't normally mind if I'm really gentle with it. You can find it at most Tractor Supply stores.
Avoid using peroxide, and try to keep her from moving around. Does she seem to be in too much pain?