Fudge - although I haven't gotten to the espresso stage of my own personal coffee experience yet, my understanding is that you want to go dark (like Italian roast dark) for espresso. Dark roasting cuts the acidity of the brew way down, which is good for espresso (bright espresso = not good).
Can't help you with shroud designs. I use oil lamp chimneys on my Original Popperies, and just let the chaff blow out the top. Of course, I only roast outside and we don't get a lot of cold weather in Houston, so I can get away with that. I've found that I can't hit second crack in the Poppery II's I've tried if the weather is below about 50 degrees, but I've never had a problem with the Mark Is. The roasts take do take longer in cold weather - I can hit second crack in 6 minutes when ambient is in the 85 degree range, but that stretches out to 9-10 minutes when ambient is around 40 degrees.
I've found that I can increase the roasting chamber temps (and hence speed up the roast) if I partially cover the top of the chimney. I use a metal measuring cup with a fairly long handle, and hold it over the top of the chimney just until I hear a response in the fan motor. You really have to listen for second crack when you do that, though, because it can get rolling in a big hurry...