Ikea's UPPTÄCKA briefcase retails for $19.99. This instructable shows how to make a detachable bicycle mount for it with a few bolts and washers and a coathanger. You can hang your briefcase on your bike's rear rack while you ride, and pop it off and go once you arrive.

Step 1: Ingredients

Parts:
1 UPPTÄCKA briefcase
1 standard wire coathanger

Mounting hardware: (see step 3 for images)
2 mounting machine screws or bolts around 3/4 inch long and about 3/8ths thick, with nuts
4 lock washers - preferably the spiny kind that grip into whatever the bolt is compressing
4 large fender washers
2 nylon spacers, about the thickness of your coathanger wire (you could probably substitute a stack of small metal washers instead)

Tools:
Big pliers for bending coathanger
a drill
screwdriver, for tightening mounting bolts

Step 2: Transform a Clotheshanger Into Pannier Mounts

A standard stiff wire coathanger will do. You're going to use it to create mounts on your bike's rear rack. For this you will need some big pliers - channel locks and a pair of normal ones in concert will do. Some heavy wirecutters are nice as well.

Start out by unwinding your coathanger, and cutting it in half. Take one end of a piece and place it through something towards the rear of the rack - a hole preferably - and then begin winding the wire around the side of your rack. Take a few turns for strength.

Next bend the wire so that it extends outward from the rack - perpendicular to the vertical plane of the bike - about 1/4 inch. This will provide room for the mounting bolt head on your suitcase to slip between the rack frame and the mounting wire. Now bend towards the top of your U, and give at least 3/4 inches for space. Create your U shaped depression. The deeper the U, the harder it will be for your briefcase to bounce out of the mount.

Repeat the same thing for the front mount.

Step 3: Add Mounting Bolts to Your Suitcase Pannier

Now we'll add some mounting bolts to our suitcase pannier. First you'll need to mark where they should be placed.

Stand on your bike and have an assistant hold your suitcase against the mounting brackets on the rear rack of your bike. Pedal (backwards, towards the freewheel) to see where your heel travels. Your assistant should find a position for the suitcase where there's plenty of clearance between your heel, while pedaling, and the leading edge of the suitcase. Be sure to leave an inch or two extra, in case one day you decide to cycle in extra-long clown shoes. Once you are satisfied with the positioning, make a mark on the suitcase in the center bottom of each of your mounting bracket Us. These are where you'll drill holes to place your mounting bolts.

If one of your holes must be drilled through the zippered pouch on the back of the UPPTÄCKA, cut out a small square from the pouch with a razor knife before drilling. Otherwise, the fabric of the pouch will wrap around the drill bit and tear.

Once the holes are drilled, insert the mounting bolts and tighten the nuts on the inside of the suitcase. See attached images. The order is:

Bolt (head)
big fender washer
nylon spacer (or stack of small washers equal to thickness of your coathanger wire)
2nd big fender washer
grabby lock washer
-wall of suitcase-
another grabby lock washer
nut

Step 4: Put the Pannier on Your Bike!

This may require some finessing of coathanger mounts! If your coathanger Us are not straight, they may not fit between the fender washers of your mounts. Straightening them can be tricky. I found placing the U mounts in a vice or screw clamp and then tightening it down helped make them straighter.

Enjoy your ride!

Author's note: I like collaboration. If you build these panniers, and discover an improvement, by all means share it in comments. If it makes sense to we can make this a collaborative instructable. Open source software gets better by incremental improvements shared by lots of people. Why shouldn't an instructable work the same way? :)