There are meny ways you can build drum pads yourself. One of the best methods, as far as I know, is converting Remo Practice Pad to electronic pad. You can also find some great ideas on the gallery page.
There are two main options. Using the drum head that comes with pad, you replace the foam inside with piezo sandwich :-) That conversion you can find elsewhere surfing the net. Other posibility, that I will describe here, is using mesh head. Both options offer good results, but mesh heads are much more quieter.
You will need:
1. Cut plastic tubing to aproximate match the length of pad perimeter. If you used wire, pull out cables first. You need only plastic. Wrap the tubing around the rim of practice pad and cut to fit.
2. Glue it with superglue to form a circle. Resulted circle should be of same size as practice pad's original head rim.
3. Cut two pieces of fiberglass mesh. Left about 2cm around the edges. Cross them to form 45º angle, like on picture.
4. Fix them together with tape. Put the original drumhead on top and mark a circle around it.
5. Put resulting tube circle on top of mesh and fix it with hot glue in about four points.
6. Put mesh+tube on practice pad, as it was a drumhead, and apply hot glue all over the circle, from above. Make sure hot glue is penetrating right down to tube.
7. Cut out around rest of mesh material at about 2cm from tube.
8. Wrap it around tube and glue with hot glue. It will form a rim of our mesh head. Let it cool down for about an hour before next step.
9. Put it on practice pad and mount it's rim. You will have to tension well.
10. Done. This is the result.
IMPORTANT: In order to get clean triggering, without annoying crosstalk between pads, pay special attention to mounting side of pad. Try to isolate piezo element from incoming vibrations through rack.
Browsing the internet news groups time ago I found a post describing how to make a e-pad from Remo practice pad. Basically, you have to dismount it and change the foam disc inside with two foam discs of half of original height with a piezo sensor glued on a thin metal disc between. Here I reproduce the most important part:
Date: Wed Aug 12 08:47:11 1992
I have a roland Octapad and wanted to build some triggers for the extra inputs. I bought the radio shack piezo buzzers and built some pads that have worked real well. The best ones were model number 23-073, cheap ($1.49 each) and simple to break out of the plastic case. I'm sure they would work equally well with the D4. For the Toms and snare, I bought Remo practice pads. They are made of grayplastic, have a regular drum head on them and a pseudo tensioning ring. They are about 2" thick and are filled with soft foam rubber. I mounted the piezo buzzer to the underside of a .06 thick circular aluminum plate with a slightly smaller diameter than the inside of the practice pad, and then cut away most ofthe foam rubber in the drum pad so that the plate rests on a foam shelf around its perimeter. The foam looks like a foam ring, with a cross section like this:
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With the aluminum plate cradled in the foam ring, there is very little chance that hits on adjacent pads can trigger the sensor. I then fill the gap between the plate and the underside of the drum head with a sheet of foam rubber (I used the pads that are for muting drums, wet suit material would work GREAT, so would your basic mouse pad). Drill a hole for the cable to come out, assemble the whole thing, and it works GREAT and doesn't look bad either. They sell these pads in 6", 8", and 10" diameters. They come with a mount on the underside, so they are easy to mount. The stick response is very much like a real drum, and is a little adjustable for your liking. Total cost to build one is between 27 and 35 dollars, depending which diameter pad you buy.
The key to increasing the sensitivity of the piezo transducer is to have it in intimate contact with a larger metal plate. The metal plate will conduct vibrations from a considerable distance to the crystal. You ARE removing the piezo transducer from the housing, aren't you? You HAVE to at least remove the bottom cover of the housing (which is the back side of the little circuit card - DON'T break it!!) and flip the sensor upside down in the housing, and pot it flush with 5 minute epoxy. You MUST have vibration thru a solid medium transmitted directly to the little brass plate that the sensor is mounted on, and the more "solid" the medium is the better. For instance, 5 minute epoxy works better than 24 hour epoxy becuase it is more brittle and has a lower damping constant.
Here you will find plans for building pads and more: EDrum For Free