Not lots of time to reply here as I should be running off to work, but there are a few obvious things you could do to make the costume more effeminate. Putting some thought off the top of my head into what Irvine looks like (and, yes, I have spent some time staring!), I would suggest a tighter blouse with a V-neck -- since his shirt is low-necked anyway -- and wearing low-rise pants instead of having them go all the way up to your waist.
I would keep the chaps, wear high-heeled boots, and on the coat, depending on if you're going to make it or buy it, maybe you can do something effeminate with it, like pulling it in to define it more at the waist instead of just letting it hang straight and loose.
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