Here is an easy and small USB remote that allows you to trigger up to 4 cameras simultaneously. I built it for SDM use. There are only 3 parts.
1. Mini-usb hub
2. Microswitch (Radio Shack)
3. N Battery Holder (Radio Shack)
The mini-usb hubs have 4 female USB ports, and all of the POS and GND pins are shorted. Only the data lines are routed through the switching circuitry (we don't use those). So with very little modification, you can add a switch and a battery to the mini-usb hub.
Start with the cheap min-usb hubs with a built in place for the male connector. I got mine free from a vendor, but you can find them on Ebay for $3. Then cut out the male USB connector. Next is the hard part. You have to carve out the plastic to make room for the battery holder, and cut a hole for the microswitch. I used a hobby soldering iron/burning tool.
Then you just put the battery holder in the hole you cut, and attach the POS and GND wires to the circuit board.
I use 3 LR44 batteries for 4.5V. I used a screw to bridge the gap in the battery holder. The other nice thing is that there is a built in LED to tell you when the button has been pressed and the batteries are good.
Then you can use cheap retractable mini-usb cables to fire the cameras within SDM. I got mine at the dollar store. They are terrible for transfering data, but they work great with SDM.
The overall dimensions are 1.5cmx4cmx9cm.