OK now that we've all drooled over the Emeralds (I know I did haha)...
L_C are you dead-set on building? I only ask because there are lots of prefab enclosures out there that should work for a boa, so if you're not real comfortable building you could go that route. I'm talking about Boaphile and stuff like that; check out the kingsnake.com site and you'll find several different ones.
I can't help out a ton with pics as I don't have the cages I built anymore, but I've built some plywood & plexi cages pretty cheaply. I used plywood (half inch as I recall) and framed it with 1x1s and just glued & tacked them together. You could maybe skip the framing and just glue & tack (nail) the plywood together, but I thought the framing would give it more stability. I built mine almost like a pair of bunkbeds; I did the measuring and cutting for both cages at once so they matched, and put 2x2 legs on them, with the legs on the bottom one a few inches shorter than the cage so the legs of the top cage could nest in them. The whole thing was actually pretty stable. I even used plywood for the tops with a small screen area for the basking lamps (these were monitor cages), with a boa you'd probably skip the screen altogether and just heat with tape/mats/panels whatever. I used sliding plexi doors.
Things I would do differently if/when I build more - I'd probably lay a sheet of linoleum or some such over the floor and seal it. The plywood floor didn't do so well with damp despite me putting several coats of seal on it; my sav enclosure held up fine but the damper Dumeril's habitat showed some bowing in the floor. I'd also frame the doors from the get-go, you'll want to do this because the animals will push out the plexi. My sav could do this with relative ease. So if you're planning hinged doors frame them all around and put a good latch on it; a boa should be heavy/strong enough to push his way through. As I recall my cages were 8'x3'x2.5'; I'd probably go 8x4 if I did it again but for a boa 8x3 ought to be more than fine.
