Step 1: Scrounge up some Material
Cardboard Enough to surround your laptop...and then some. Any box big enough should do.
Duct Tape The Force that holds the universe together...in any color(s) of your choice.
Foam Packing Sheet This will be used for the padding...bubblewrap might also work if done carefully.
And you'll probably want these tools too:
Razor/Box Cutter For cutting the cardboard...not your fingers.
Yard Stick Very useful for measuring things, though you could make due with a 12" scale.
Marking Implement Pen, Pencil, Marker, Quill, Charcoal Stick, etc.
Note: Scissors could probably be used in place of the Box Cutter
Oh, and a Laptop of some sort to put in when your finished...
(Sorry for the fuzzy pictures...I didn't find the 'Steadyshot' feature on the camera till later)
Step 2: Measure once, cut twice...
My Eee PC fit rather nicely in one of the edges of the box, so that helped give me a reference for basing all my other measurements.
Basically, you want to measure out the space for your laptop and power cords (and whatever else you may want in there). And then mark out the space. The width and length of your laptop will determine the size of the 'main' area. The height of your laptop will determine the length of the tabs on the sides of the cardboard piece. For the shorter end of the Cross-Shaped base, the length will be that of your 'main' area (width/depth of laptop) added to the height of the laptop. The longer flap will get cut shorter later.
It is easiest to cut out the overall rectangular shape first, and then cut out the corners if you're not using all of the cardboard.
Make sure to score along all cuts first if using a Razor/Box Cutter...this will make cutting the cardboard much easier.
When you have finished cutting everything out, it should look something like the final image. If you are making the compartment for the AC adapter, and whatever else, make sure to keep the cut out pieces. The will be used as separators for the compartments.
Step 3: Making the flaps foldable
You will want to redraw all the lines that you have on the 'Inside' of the case onto the 'Outside of the case. You'll also want to measure out the height of the laptop and mark a line for it on either side of the 'main' area. These lines will also be scored. (In the case of the line on the larger flap, another line should be scored about 1/4" or so outside of it.)
Also, now would be a great time to cut the longer flap down to size and shape. It will be the "latching" flap on the case, so size and shape as you see fit.
Step 4: Using The Force...well, Duct Tape...
At any rate, what you will want to do now, is gather up the padding material that you're going to use and lay it out. Make sure it covers the areas on your case that you want it too, and cut it to size/shape. I had strips of the stuff, so I had to kinda layer it in a way (refer to below pics). Also, make sure not to tape the hinges on the flaps in the flat position, scissors and slits in tape can be your greatest allies here. Cover everything in Duct Tape.
Also, make sure that you only tape the inside at this point, and make sure that all pieces are smooth...your laptop will be going in here.
Also note, overlapping the strips of duct tape a little bit will greatly help in this process.
Step 5: Add the Partitions
Coat the tab(s) in duct tape, and then set your laptop/items on the case and lay down your separators accordingly. Once they're set where you want them, remove the items and lay down a strip of duct tape. Fold the separator up, and lay down a strip of duct tape on the other side. It would also probably be a good idea to cut slits in the corners of the duct tape for easier application.
Once it is mounted to the base, fold up the bottom of the case and attach the duct tape that is attached to the tab to the bottom of the case. This will prevent the separator from moving around in the case...this is also probably the most tedious step in this entire instructable in my opinion.
After it's attached, fold up the sides and tape them all together. You now have a rudimentary laptop case. I suppose you could stop here, but then you wouldn't have the Snazzy Duct Tape Finish.
Step 6: Duct Tape Taxidermy
Basically, this part is up to you. You can put the duct tape on in horizontal, vertical, circular, random, etc. strips to cover the case. I personally used horizontal strips to cover the case evenly and to provide a base for the detailing.
Tip: Cut slits in the duct tape wherever there is a folding piece or corner, this will make for a nicer looking finish if you're into that sort of thing.
Note: Make sure when going over the folding flap with the tape that you put the tape on with the flap folded past the farthest it will be folded over when closed. This is done to allow the greatest degree of freedom for the flap to move around in.
Step 7: Detailing and Finishing
The final step is to get something to cause the flap to latch, Velcro, Snaps, Buttons, String, Elastic Bands, anything but magnets ought to work.
And now, your case is finished. If all materials are present, this shouldn't be more than an evening's worth of work. Load up, and get ready to show off your geeky ingenuity!
Note: This was my first Instructable, and I hope you found it useful.