I mean, Technically it works, because the only blocks that the room checks to make sure is letting in light to give the greenhouse effect is the ceiling, the walls can be solid dirt for all the game cares. And, if you wanted to do more of a chiseled smother diagonal, you can cut out the 8x8x8 corners on the inside and outside of the extra blocks. idk, i haven't built a whole structure, but, something like this seems to me like it would work, looks half decent, and still fits the "50% or more" criteria, if my math was correct on what i was taking away.
tho, i am just kind of necro posting at this point also on a post almost a year old //media.invisioncic.com/r268468/emoticons/sad.png