OK I am totally new to the care of reptile eggs but I have been hatching chicken, quail, pheasant. ducks, and turkeys from eggs since I was a little boy (some 25yrs ago I started to help my grandma). I have several home made incubators and honestly I trust them more than any of the store bought bators I have ever owned and I used to have a couple of really expensive ones.
Now I could better explain all of this if I knew more about whay it is you are needing to achieve in temp and humidity from your bator, but here goes what I have done and has work in the past. Honestly I have never had to insulate any of my incubators if anything I have had too much heat and needed a way to find a way to let it out. The computer fan is an excelent choice for air circulation. I have a very large cabinet bator that I can hatch 100 chicken eggs at a time in that I have used 2 fans for and they circulate air like a dream, they will also help to keep the temp constant in the entire unit from top to bottom which is actually the purpose behind it. Now as for humidity I use a milk carton cut in half with a couple sponges laid in so they are sticking out of the top, this gives more surface area in the air surrounding to help evaporate the water and thus make it much more humid. For heating I have never needed more than a couple of light bulbs (except for my large cabinet bator..... it's really big) now for my small quail bator (I normally hatch 4 dozen at a time in here) I have 2 of the compact florescent bulbs 60 watt equivalants. This at first actually over heated my incubator a bit to much so I had to drill a few small vent holes in the back to allow some air out and thus in as well to cool it off to the propper 99.5 mark, this messed with my humidity a bit so I just added another sponge to the water jug. and I got it back to the 50%-60% that I like for the first 18 days and then the last few days I want it at 70%-80% and if I add 1 more sponge then I get normally about 75-77 and I'm golden to fiinish the hatch. When I first set this system up I had temps over 120 with 2 100 watt equivilant bulbs that is why I dropped to the 2 60 watt bulbs and I use the day light bulbs that simulate natural sun light and since they are the compact florescent bulbs they use much less electricity and I have been running the same bulbs for 3yrs now with no issues at all (hatched probably 30 or so clutches). Like I said I did not insulate this bator and made it simply out of some half inch OSB from the Menards with am acrylic door so I can see inside and then I plumbed in with some cheap PVC tubing so I can fill the water bucket for humidity and then also one so that when they start to pip I can fill the water dish so they can have a drink. I hate to open the door for anything so I can rotate the eggs with a stick stuck into a hole and then fill all the water without opening the door at all.
Now like I said I am not sure of the requirement for hatching snake eggs so I may have wasted a ton of breath here or I may have helped some I really don't know.... If I did help and you have any further questions just let me know I have built far to many incubators to not be able to figure this out with you.
Steve