Alright kiddies.....can it.
On the issue of cornsnakes...
1.) Though cornsnakes occasionally hit the 6 ft marker in nature, its not that common in captivity. The largest I've ever seen barely exceed 5 ft. That said, the general average of the species in captivity is around 4.5 ft.
2.) Without delving into the issue of reptiles "being unhappy" and whether or not they're even capable of the emotion, many keepers house adult corns in tubs with roughly the footprint of a 15 gallon (24" x 12") and their animals fare just fine and thrive and produce year after year. While I'll admit that colubrids are a tad more active than say, a boa or python, they still rarely make use of a spacious enclosure and are likely to be content with the "basic" needs in a smaller, cozier set-up: loose substrate for burrowing, couple of hides, a branch or two for the occasional climb and obviously a water dish. All that combined with regular handling should give any cornsnake adequate exercise and I highly feel it would feel "cramped." If the glass tank in question is 30" long by 12" wide, thats perfectly fine for an adult corn.
I see the original poster is leaning towards a sand boa, but I wanted to insert my $.02 about the corn snake discussion.
Louie, how many sand boas have you personally encountered? To say that "most will bite" is a pretty substantial claim, and a foolhardy one if you do not have personal experience to back it up.
However, you're correct in stating that except when they're eating or being handled, sand boas are actually quite secretive, and consequently boring captives. One rarely sees them inside their cage as they stay burrowed 90% of the time.
Here's a few more colubrid suggestions:
garter snakes: requires no extra heating, eats variety of prey items, can be a little highstrung
African house snakes: requires a little heating, very hardy, very tractable