I had a couple of pieces that should fit together. But they didn't. I tried to file them, but my file was too big and the holes that should be sanded weren't deep enough for the file.
The obvious solution was to use a dremel like tool, but I lacked it.

The following steps reproduce the steps I took to make a last resource tool... I took the photographs in reverse order, when dismantling the tool, after successfully using it.

Step 1: You'll Need...

You'll need the following tools and materials:

* Some tape. I used masking tape, but duct tape should do it.
* Sanding paper. I cut a 1"×2.5" (25×70mm) piece.
* A ?socket drive extender? I took it from a ratchet set. If you have a non-percusion electric drill, you could use their bits.
* A electric screwdriver. I used a cheap screwdriver with fixed speed (that's all I have). Maybe a drill could do it, but you should use it on the slower speed to avoid accidents.

Step 2: Cut the Sandbox Paper. Protect the Extender.

Start by cutting a piece of sanding paper and protecting the extender with some tape.
Then make a cone with sandbox paper.

Step 3: Fix the Sandbox Paper.

Attach the sandbox paper to the driver extender and fix it with some tape.

Step 4: Strengthen the Structure.

Strengthen the structure by adding more tape.

Step 5: Use It

Attach the extender to your electric screwdriver and start sanding.
Use the most caution and (if possible) low speed. Remember your "tool" is unstable and can do nasty things like jumping over the pieces (destroying them), jumping to your eyes or fingers, or even catching fire if it gets too hot due to friction.
Also, use protective mask and glasses, like you should with a real dremel-like tool. This thing produces small particles that could seriously damage your lungs.