Olive's Second Halloween: Planning

Originally, I planned to make Olive a little Iverness cape and Deerstalker cap out of herringbone stash fabric for Halloween- obviously she was going to be Sherlock Holmes. I even planned to have a red buttonhole. But then a bug hit and I started shopping for red suiting and yellow fabric. Why? Olive is going to be classic Carmen Sandiego. I loved 'Where in the World is Carmen Sandiego?' and had the Where in Time computer game which I completely wore out.

Everyone loved this idea and so I have started another costume.
So here are the parts for her costume:

The Trench - I already drafted an un-tested Iverness so I need to omit the cape and draft sleeves (uuuugh, hate sleeves) and figure out the lapel situation. I know shades vary but I think a bright red would be good to be more eye-catching, but I don't want it to get too McDonalds-y.

 I'm thinking a Red Gab, it'll be lightweight enough to not bother Olive but enough structure for the trench.

The Shirt- I'll throw together a yellow dickie so Olive's not wearing too many layers. Maybe make a cowl neck or something

I'll go cheap and use lemon broadcloth. I could make a slinkie/sleazy-type-thing out a knit but I'm not sure I want to do that or not.

The Hat- I'm not how dramatic to make it. Olive's great about hats, as along as she can see, she's good. If I had my way it'd be HUUUGE- let red hat society huge.

I'm not sure which is the best method for Olive's hat- make my own felt hat (with an itty bitty crown) or piece a fabric one together. I'm thinking piecing might be better for my own sanity but felting would look cooler. DH wants a big hat too - but his contribution to sewing is ironing so there's that. I'm not sure if I should use ribbon or fabric band.

I made a quick sketch of what I was thinking but it just looked like puppy as a farmer, I hope that's not an indication of how this is going to go.