A while back my laptop decided to have a heating problem that resulted in the fan being on most of the time. This should probably be looked at but in the meantime, I made a stand that would allow air to circulate beneath it. There are no moving parts, no power needed, and it's light and portable. I can't actually give you an estimate on how efficient it is but my laptop usually seems happier when I use the stand.
If your laptop has a very serious problem, you might want to look into getting a stand that has a built in fan (or get your computer looked at. I've also heard that spraying compressed air into the fan can help), but if your problem isn't too bad, I recommend this stand.

Estimated time: ~30 Minutes
Estimated difficulty: Easy
Estimated cost: $15*

*Per the amount of material you need for one stand if you use my design exactly. I used what I had laying around so mine was free. What I recommend you to do is to use the basic shape of my design but replace the parts with whatever you have already.

Step 1: Tools & Materials

Materials:
Acrylic, 9" x 12" x 1/8"*
Just needs to be a stiff, strong sheet of something. Wood and stiff cardboard would probably work too.
Thick-walled rubber tubing, 18" long, 3/4" diameter**
Just needs to be something that can hold the laptop up at least half an inch
Small machine screws/nuts/washers, 4 sets. Plastic will not scratch tables as easily and will slide nicely. If you don't want sliding, use rubber pads on the bottom. If you don't have washers, use slightly larger nuts that will slide on the screw.
Just something to hold the tubing to the plastic

Tools:
Bandsaw, or something else to cut plastic
Sandpaper to make the plastic edges smooth after you cut them
Drill with drillbit the size of your screws
Knife, or something else to cut rubber
Pliers/Screwdriver, or something to tighten the screws with

* These dimensions are whatever the bottom of the laptop is
** This dimension is twice the length of the laptop. When I say length I am referring to the distance from the front of the laptop to the back. Technically you can use either the length or width, but the length works better, as I will explain in Step 3.

Step 2: Cutting the Plastic and Tubing

Cut the plastic to the correct size and sand the edges. If you can, also cut/sand the corners off so that you won't have sharp corners that dig into you. If you have access to a belt sander, you can do what I did and cut sharp corners and then just sand them into rounded ones by turning the plastic as you sand. This is not an exact science. Just make them round.

Cut the tubing to 9" pieces

Step 3: Drilling the Holes

You will be drilling holes in the plastic and tubing. You can either have the tubing going "vertically" or "horizontally". Vertically would be towards and away from you, horizontally would be side to side. Vertically is better because there will be less stress on the screws if you tilt the laptop towards you, like if you're holding on your knees.

You will put two holes in either tube and two corresponding holes in each side of the plastic for a total of two holes in each tube and four in the plastic. Make two holes in each tube about 1/2" fromt the end (see note at end of step). Now make corresponding holes in the plastic, about 1" from the side edges. Because nothing here needs to be exact, I would (and did) just line up the tubing with the holes and see where to drill. Once you're done, make sure all the holes line up.

** Make sure the holes are not so far into the tube that you can't reach them to put a washer/nut on them **

Step 4: Laptop Cooler, Assemble!

The difficulty of this step is dependent on whether or not you have washers (I didn't). What you're going to do is put the screw (shaft first, so the head is against the bottom of the plastic) through the plastic and and tubing. Whatever you're using for washers, put them on the screw after they have entered the tube. Then put on the nut and tighten. The order of layers, from bottom to top, should be:
Screw > Plastic > Tubing (one layer) > Washer > Nut > Tubing (other layer).

The reason this step is annoying is that it is hard to place/tighten the washer and nut inside the tubing. That's why the holes in the tubing are so close to the end.

Done

Step 5: Conclusion

I've been using this stand for a couple of months and have had no problems with it breaking or otherwise getting damaged. I keep it in a backpack and that seems fine.

Quick note on design in general.
I started this project with the idea of a folding stand that had hinges, a lock and was all plastic. Over a little sketching (always sketch everything forever), I realized simpler and simpler designs until coming to this one. Although it wasn't as cool as the one I originally thought of, it was a better, simpler design. Sometimes that is the case and it is important to recognize it when it does.