I made this little cat cave for Plop with some very simple materials I had around the house:

Here's how I did it:

Materials

Here are the materials that I used:

a cardboard box
a T-shirt
packing paper (optional)
a utility knife (optional)

box and shirt

The cardboard box should be one that you know your cat fits in. Make sure that your cat can comfortably fit through the neck hole of the T-shirt.

Plop really likes the crinkle sound that packing paper makes. I lined the inside of the cardboard box with the paper so he'd enjoy his time inside. The utility knife is for removing the flaps from the cardboard box, if you choose to do so.

HOWTO

1. Prepare the box.

Prepare the box by either:

folding the flaps inward.
using the utility knife to remove the flaps.

I chose to remove the flaps entirely to make the box simpler and to give Plop more room inside.

2. Optionally line the box with packing paper.

I folded packing paper and put it on the bottom of the box, so that Plop can sit on it.

3. Cover the box.

Cover the box with the T-shirt. Position the neck hole so that it's over the open part of the box.

4. Tighten and secure shirt.

To tighten and secure the shirt, fold the sleeves inward. On the opposite side of the neck hole, tie a knot to tighten the T-shirt around the box.

5. Present box to your cat.

Present the box to your cat for evaluation.