I recently spent some time at my parent's place. As I opened an old box I found one of those toy echo mics.
This gave me the idea to build a small homemade spring reverb!

But how does it SOUND!?!?!
Glad you asked. Like this. (soundcloud link)

Step 1: What You'll Need

Stuff:
- echo mic toy ($2)
- little amp & speaker (I got mine from a 1-watt Marshall amp I had laying around) ($35 for the Marshall one I believe)
(you could of course use something more DIY like a little Kemo amp for example. Make sure your speaker fits the specifications of the amp)
(You might want to get an amplifier for after the piezo mic as well. I don't have it- I just use the instrument inputs on my sound card and it works fine. But in situations where you don't have gain, it might be useful.)
- In the case of the Marshall MS-2, an extra 9v battery connector ($0.5)
- jack socket (you might need two if you're working with a more DIY amp.) ($0.5)
- a box in which it will all fit. So speaker, amp, spring, etc. will have to co-exist in this space. My box was around 18x13x7 cm.
(make sure the box can close with everything in it. Mine can't fully. It's no biggie tho. ($0)
- 9v battery to power amp
- piezo element ($1)
- 2 small paperclips/metal hooks/thingies to glue to speaker/piezo so you can attach the spring.

Tools:
- Soldering iron!
- Glue gun!
- Drill!
- Saw!

Step 2: Saw Open the Mic, Get That Spring!

First you'll need to cut open the echo mic.

I used a saw for this and just sawed along the side until I could get to the insides.
The spring might be stuck inside, but with some force on the outsides of the mic, you should be able to get one side to detach.

The other side however, is a bit tricker.
I used a pen and a paperclip to kind of "fish" the other end out.

FInally, you'll have a detached spring! :)

Step 3: Prepare Your Amp...

So you'll either be removing the amp from something like the Marshall MS-2, or putting together your own amp.
This is pretty straightforward.

Test it when you've taken it out.
I had to resolder the battery compartment to a more flexible one.
Ignore the pot in the picture.

Step 4: Prepare the Contact Mic + Speaker

So, we'll need something to connect the spring to.

I glued (with my gluegun) SMALL paperclip-like objects to the conus of the speaker and to the contact mic.
See pictures.

Make sure it's sturdy and dry before you continue.

Step 5: Solder Piezo to Jack Output (or Another Amp With Output)

Solder your piezo element to a jack output...

Step 6: Glue It Into the Box!

First test if your spring will reach both sides, then glue the speaker and piezo into the box!

Step 7: Test If It Works

Do a quick test if it works! Connect the spring to the hooks,

Hook up something that makes sound to the amp (like a little keyboard/synth/whatever).
Make sure the spring can vibrate freely, and the glue will hold.
Connect the output to an input on your audio interface, preferably with instrument inputs.

Go wild!

Video of my prototype:

Step 8: Finish It Up

When it works, you can continue.

Drill holes for the pots of your amp, jack input/output, light (if your amp has one).
Glue it to the inside of your box with the glue gun.

I extended the wires for the battery compartment and glued it to the top.

Then, you're done! Only thing you'll need now are some nice knobs for the pots. But I still have to find those for mine as well...

SPRINGGGGGG!

How it sounds. (Soundcloud link)

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If you like it, please have a look at my other instruments, sounds and compositions at: http://www.markijzerman.com