I always use my laptop on my lap, and it tends to get really hot. I looked online for some kind of stand, but they were all really expensive. I made my own out of a scrap of plywood and some rubber. Now the laptop sits nicely on my lap and still has airflow around it.

Step 1: Gather Materials

I used a piece of quarter inch underlayment plywood that I found in the garage. I then used a scrap of industrial grade rubber that was lying around.

Step 2: Measure Twice, Cut Once

I measured the dimension of my laptop, roughly 12" x 9". Then I sized the plywood a little bit smaller, just because I wanted a smaller profile with the stand.
After I had my piece of plywood I eased the edges with a router to just to make it a little bit nicer. I then used a razor knife to cut out six pieces of rubber. I sized them to be roughly a square centimeter each. I made six so that the back would be a little higher, giving me a slight angle with the laptop.

Step 3: Glue

Next, I flipped over the laptop to find good spots to land the rubber feet. I decided to have it rest next to the laptop's feet. I marked the spots on the piece of plywood.
I used E6000 glue. It is a "craft" glue, but it will glue anything. I glued the two stacks of rubber first, then I glued all four down to the wood. Within 24 hours it was dry and they were solid as a rock.

Step 4: Test It Out

Once it's dry, put the laptop on it and see how you like it. You can always pry the rubber off and re-glue it. I didn't need to cut any angles on the rubber, becuase it squished down a little bit with the laptop. I had to scoot the laptop forward a little bit for balance issues.
It works out great, and it fits right in my laptop bag, too. The laptop stays a lot cooler with just a little bit of airflow around it.