Don't you hate how most arduino enclosures you find on the web cost about half as much as an arduino itself? Some enclosures cost almost as much as an arduino.

Here's how to make an arduino enclosure for under $3

Step 1: Buy Stuff

Go to Lowes or Home Depot and buy a Carlon 1-Gang plastic Old Work electrical box the model number should be B108R
here's a picture of what it looks like http://images.lowes.com/product/converted/034481/034481043768xl.jpg

next get a Raco metal electrical box cover
that looks like this http://images.lowes.com/product/converted/050169/050169008607xl.jpg

then get two pairs of nuts and bolts that fit in the two main holes for the arduino
optional:
you might also want a couple extra nuts if you want to lay your box flat on something

note that you might already have nuts and bolts that fit already in your garage
hey, if you're lucky you might even have all of this already
but if you don't,
all of this shouldn't cost much, about $3 or less

Step 2: Tools You'll Need

Most of the work will be done with a drill and various drill bits
a vice will help to hold it while you drill

optional:
a nibbler tool is good here too but not needed
a dremmel is fine too
hack saw and glue might be used

Step 3: Ok Start!

Put your arduino inside the box to see where you will need to drill holes (you will notice it fits in quite well)
eyeballing it is ok
realize that there is a bump in the plastic which will make the arduino teeter
you should see how you like it before drilling
don't worry about messing up, it's a cheap box

if you have a nibbler, drill a hole to fit the nibbler in, then cut out the square shape for the usb
even with a nibbler you may still mess up but it's ok

note on the finished picture you will have to also drill a hole on the bottom of the center indented part to get the arduino flush against the box
this also requires pulling some of the plastic off once you've drilled ( a dremmel would work well for doing this part)

Step 4: Drill Holes for Bolts

Once you have the arduino situated how you like it (probably with the usb port protruding out and the pcb somewhat against the plastic wall), you should make a mark on the inside where the holes are

drill holes that are about the same size as the holes in the arduino
once you have the holes put the bolts through and tighten the nuts on (not too tight)
you could also use spacers if you want

if you're making something that's going to be a hand held device, you probably don't need to do anything else with the bolts
but if you're making something that's going to sit flat on a desk you might want to mark the bolts and cut them of with a hack saw flush to the nuts, then glue on two other nuts at opposite sides of the bottom of the box to make it sit flat
or you could also use bumpers if you have those

Step 5: Pretty Much Done!

That's pretty much it

you can see that there's room for most shields (a couple cm) or other electronics

the metal cover allows you to either just cover it or to use for panel mounted interface parts