Build a water resistant box with cooling fans inside.
Wood is probably the best material for you to work in.
Make a box that is bigger than the notebook. It should have a 1″ clearance on the front and back and a 2″ clearance on the sides. Make sure to allow room to put your notebook power transformer inside the box. Put it on the front of the notebook so it won’t combine with the heat produced at the rear of the box. Don’t put a hinged lid on the box; instead make another box with 1″ sides (the lid). The lid box should be just large enough to fit snuggly on top of the box. The lid box will help keep water out. The bottom of the box should have small blocks 3/4′ tall, by 1/2″ wide and 2″ long. Put in three or four rows, to support the notebook, and make sure that you have space between the blocks so the air can circulate. Glue some Velcro under the notebook to mate up with Velcro on top of the blocks. Use the soft side on the notebook. The main path of circulation should be on the sides so the 1/2″ wide part should face the sides of your box. Run the cables from the back of the notebook to the front of the box, which faces down slope. The holes for the cables should be 1″ from the bottom of the box then water won’t get in, and they can act as vent holes, so oversize the cable holes in the box. The up slope (back) of the box should have no holes in it so water can’t get in. You can add a few holes, on the front, near the bottom of the box on the down slope side so it can drain out any water that does get in. If water does somehow enter your box it will seek the bottom of the box and try to flow down slope. The small blocks that elevate the notebook and the drain holes on the down slope should be able to drain all the water out and keep your notebook dry.
The most important factor will be the heat. You will want to mount a muffin fan on each side of the box near the rear of the computer, where the most heat is generated. The air channels you create under the box should run from side to side with gaps to allow front to rear air movement. You can power the fans with a USB connection to your notebook. Or you could include another transformer at the front of the box that will provide DC voltage to handle the fans. If the fans are blowing out then water can’t get in. Check the height of your box so you can fit the muffin fans into the sides without having them too tall for the box, or blocked by the lid. Add a few more vent holes on the side of the box, near the front of the notebook and on the front of the box. These vent holes should be 1 1/2″ down from the top so the lid won’t block them. Make these additional vent holes only be 1/4″ diameter so it will keep out most insects.
Paint the box and lid then seal it (inside and out). I like to use polyurethane; it will seal the box in plastic that is waterproof. It will also create a shiny surface that can reflect heat. Then you can select a paint color that matches your roof our your equipment.
When you complete the box put in a couple of Raid ant traps to handle any intruding insects. Ants like warm places and they could get to your roof.
Attach the box to the roof with construction glue. DO NOT nail or screw them to the roof. You will create holes in the roof so water can get in and damage your roof and ceiling. The box should always remain on the roof near your other stuff. You can easily pull out the notebook by just removing the lid and unplugging it.
Check out American Scientific Surplus: http://www.sciplus.com/search.cfm
To find the fans and maybe an AC to DC transformer to power them.
You can get fancy and include a temperature sensor to control the motors, but I would just leave them on all the time. You could also find these at American Scientific Surplus. You could even include some small blocks on the bottom of the box to lift it off the roof. If you do that make sure these blocks have their long dimension running down the roof slope so the minimum of water will collect on them.
Most of the other supplies can be found at a Home Store like Lowes, or Home Depot. You can use the cheap particleboard (a combination of glue and wood chips). Or pay a little more and use real wood plywood. Both will be strong, but real wood will be a little lighter. You could get all, but the wood at a Walmart Super Center.
All measurements are approximate, adjust them as you desire.