Here's a quick and easy way to make a lightweight embroidery hoop out of a plastic take-away container. It only took me about an hour for my first try, figuring it out as I went.
I used my completed hoop to embroider something for another 'ible, please check it out too!

Step 1: You Need...

A plastic container with a lid that 'clicks' into place. E.g. a take-away container, or a small ice cream tub container
A Stanley/Utility knife, or an X-acto knife. Any knife strong enough to cut through plastic
Strong pair of scissors

Some other optional stuff:

Nail File/Sandpaper (not shown)
Duct tape (not shown)
Wire/pipe cleaners

Step 2: Cut the Container Like So:

In the first photo, I marked the containers with a red pen to show where you should cut. Basically, cut 1/2cm in from the raised edge of the lid (1/5 inch) and 1/2cm from the raised edge of the container.

<---!SAFETY NOTE--->
Use the knife, and take lots of care! If you're really young and nervous, or even if you're not so young and nervous, I suggest that you delegate this job to someone with steady hands.
Also, make sure you're cutting on a protected surface, so on lots of newspaper or a thick magazine.

Step 3: Filing and Duct Tape

The next, optional step is the file the edges smooth and/or cover them with duct tape.
It is kinda important to have smooth edges that won't catch on your precious fabric, so I do suggest one or the other.

Step 4: How to Use?

To use your new embroidery hoop, take the lid apart from the bottom.
Place your fabric face-up over the bottom piece, positioning the part you want to work with in the centre.
Then place the lid over the bottom. Push it into place, until the two pieces 'click' together.
The fabric should be pulled taut as a result of this. If not, pull the fabric taut as you click the lid into place.

Take into consideration that this is only a very light embroidery hoop, and it can only take very thin fabrics.

I did have an idea of cutting the lid at one point, then threading wire through the lid, so that it would be more like a commercial embroidery hoop, but I didn't actually try it. I'd be interested to know if that'd actually work.

Thanks for reading, and I hope you've enjoyed!