DC Adapter
Making an adapter to run your Thingamagoop or Thingamakit off a DC power supply is very easy. This guide is based around using a standard 9v stompbox power supply like this one but any 9v DC power supply could be used.
1.
First, you'll need a 9v battery clip and a female 2.1mm DC jack. I recommend using a hard plastic battery snap since the soft ones can get stuck in the device's battery holder.
Here's parts I used from Mouser : Battery snap DC jack
2.
Unscrew the plug's plastic housing and put the wires through as shown.
Stompbox power supply plugs are positive on the outside and negative in the middle. This mean that the lug from the center should connect to the red wire and the lug for the outside should connect to the black wire. If you are using a power supply with the opposite polarity, with the outside negative and the center positive, just flip the wires.
After it's all soldered up, cover the small lug with electrical tape and use some small pliers to crimp the longer lug around the wires. 
3.
With everything soldered up, make sure the wires aren't touching and screw the housing on. Connect the adapter to the battery holder, plug up the power supply, and turn the device on. If nothing happens IMMEDIATELY turn it off and unplug the DC adapter. Check your polarity and make sure the wires aren't shorting out.
4.
For the Thingamagoop, you can use diagonal cutters (and safety glasses!) to make a small notch on the back piece. If you don't want to perform surgery on your noise buddy, you can simply leave the bottom 2 screws out and have a gap between the front and back.
For the kit, drill a hole big enough to pass the plug through on the back or side.
With either device, keep the adapter inside. Only the power supply's cord should be outside the box.
If you'd like to purchase a pre-made adapter, they are available for $4 + shipping