They fit 5th generation, 30 gigabyte Video, and 6th generation, 80 & 120 gigabyte Classic iPods.
Step 1: Materials
2. I use a Dremel with a tapered sanding bit to carve my cases. You need this bit specifically so that you can drill, cut and polish your case. In the past, I have used a soldering iron to melt the plastic but I wouldn't recommend it.
3. Your iPod video or classic for reference
4. Needle nose pliers
6. A good rag for catching shards and fillings of the cassette case. I've found hand towels to be most effective.
Step 2: Removal of Cassette case's unnecessary plastic structures
Then, Use your pliers to bend and break out the structures on the black half of the case which ordinarily brace cassettes. Leave the structures hugging the side wall since they can be used to brace your iPod
Next, use your Dremel to sand down the broken edges. Use your fingers to test smoothness, iPods scratch easily.
Step 3: Marking the case
Do mark the headphone jack for cutting to the edge (so that you can open your case while using it) but do not do the same for the USB connector. I've found that this significantly reduces the strength of the case.
Step 4: Carving
By the way, bits of plastic may melt and form on your Dremel bit (in the long term damaging the bit slightly). For now, I would suggest using a strong Exacto knife to force these loose.
Next, separate the two parts and drill your headphone hole from the edge of the plastic.
To cut the hole for the USB connector, drill straight into the case and then carve the length of the plug. It is important that you use your drill bit's taper to cut the correct width.
Then, put the two parts back together again to carve room for the USB connector in the clear plastic structure next to the hole in the black plastic you've just made.
Be sure all the inside edges are smooth and then place your iPod back inside to test your holes. Plug in your headphones and USB connector and be sure that your click wheel circle is wide enough. Go back and make any alterations if necessary.
Step 5: Polishing
Sand in steady long arcs at different angles around the edge. Go back and be sure that the inside edge is not sharp either.
Test once again with your iPod inside and your finished! ...Unless you want to do something about the extra space your ipod has, then check out the optional step.
Step 6: Optional: Cushion your iPod
So first, cut your sponge to fit the dimensions of the extra space behind your ipod in the case with either a swiss army knife or an Exacto knife (scissors could probably even work). This is basically trial and error since I can't tell you how much room you'll have.
Next use strong double stick tape (I like carpeting tape) with your printed or drawn on paper to cover the sponge. I also used a black paint marker to cover the top and bottom edge which weren't covered by paper.
You can also try cutting a thin rectangular piece to pad the extra width room in the case. This helps to keep the case shut in addition to padding the case.