I started my construction by buying two of the peeps, a 180 degree with a 1/2" barrel and a 200 degree with a 5/16" barrel. Then I just held them up to the lenses of my digital cameras (carefully) to see approximately what I could get. I didn't have much luck with my lenses on the Nikon D80, but the Canon G2 (point & shoot) looked decent, so I got a 1 1/2" PVC cap at the hardware store (the P & S doesn't have a thread on end of the lens), found the center, drilled the hole and screwed the peep in:
I had to put a layer of transparent tape on the inside of the PVC for a snug fit, and screwed in the peep lens a bit to make up for the rounding of the cap inside.
I still want to get back and try some ideas on the D80, but for now this is what I had time for. It seems to work best with the P & S lens telephotoed out and set to "close-up", and with a - 2/3 stop compensation. You have to play with it a lot tho, and expect to do some clean up in Photoshop as this lens isn't quite up to Hubble specs. It's super fun experimentation tho, and made for the digital camera, where you can see it right away and make some adjustments. I want to play with cutting down the peep barrel as the back lens is in quite a ways. I also have some other lenses to combine with it. Sometime.
This is the most definitive source for info I have found so far, there you can download a 26 Meg pdf tome of some people's "research" of the idea. I have some other collected links if that's not enough. I will also post some of my experiments with it as they come out of the "lab."
Ahhhh! It's ALIVE!!!!!