One shortcoming of my Miata is the lack of interior storage. The only place you could put a phone is a small cubby ahead of the shifter, but under hard acceleration, anything placed there tends to slide backwards out of the cubby and then fall onto the floor. A common solution is to buy a product like this to mount the phone on the dashboard, but I didn't want the phone to be in my field of view and cluttering up the dashboard, and I also wanted to store my iPod sometimes.
So to solve this, I sewed some simple storage pockets about half a year ago. I've found them really useful and they've held up pretty well.
The pocket on the passenger side looks like this:

It's just a simple black canvas pocket that is attached with Velcro to the side of the transmission tunnel. I only needed to use the hook side of the Velcro because the transmission tunnel already has fuzzy carpet on it.

The Velcro I used was the "Home Improvement Industrial Strength Strips" product, with adhesive backing, cut to size. I initially wanted to sew the Velcro to the fabric, but the adhesive was really gummy and quickly clogged up the needle on my sewing machine, so I ended up just sticking the Velcro to the fabric using the adhesive only. The adhesive has proven to be very strong and is only peeling a little bit after half a year.

Here are the dimensions of the final pocket as shown above. I'm happy with this size because it comfortably holds a modern phone + excess USB cable. Any longer and it would interfere with the passenger seat; any deeper and things would take more effort to dig out.

My first attempt at this pocket was a bit smaller than I wanted for my phone. However, it turned out to be a nice size for storing my sunglasses, so I kept it and stuck it on the driver's side. Here's the sketch for that first version:

If I were to make these again, I would:
- Look for Velcro without the adhesive backing, so it could be sewn to the fabric and would never detach.
- Put the Velcro on tabs extending outside the pocket, rather than on the pocket itself—this way, stuffing the pocket full wouldn't cause the Velcro to be pulled away from the transmission tunnel.
- Consider using fusible interfacing to make a sturdier pocket, though I don't think that's really necessary.
