Jay Martin has a nice introductory article on using a hovabator on his site. Find one of his posts below and follow his link.
In my opinion, however, with the number of snakes you intend to breed, I would make you own, larger incubator. There is an article in the August '07 Reptiles that has some of the basicson incubator construction and no-substrate incubation, and it was written by a really swell forum member...
As far as your temps go, do NOT incubate at 91 degrees. I cook mine around 89 and have babies pip between 51 and 56 days most of the time. Higher temps won't do any better than that, and may be overly stressful. Not that some folks haven't done it, but I wouldn't. I hear the "farms" that hatch collected eggs in Africa incubate at 95 plus, and have all sorts of kinks and other deformities...
And finally, I STRONGLY reccomend using a modified no substrate incubation method, and humidity will become a non issue.
Good luck...