The battery goes in the slot on the back of the bracket. If you have anything larger than the 500mah LiPo, it will not fit in there. The interior dimensions of the battery holder are 31 x 5mm. The holder is 19mm deep. If you don't have that battery, you can simply take a different LiPo and use adhesive velcro to attach it to the bracket or attach it under the seat with a velcro strap, rubber band, or something else.
Once you've printed the mount, use a 1/4 20 bolt and a 1/4 20 nut to attach it to the saddle rails. I used a nylon lock nut to avoid it shaking loose, but a normal nut should work fine as well.
The mount was designed to fit most saddle rails, so if your saddle has oval rails (saddles with carbon fiber rails often do), it might not work as well, not to mention that you have to be very careful clamping anything to carbon oval rails.
There is a chance that when you attach it to the rails, it won't grip well and will be able to be moved around easily with your hands. In that case, take a tiny bit of rubber from an old inner tube or from a light clamp shim, and put it in-between the 3d printed part and the saddle rails.
Then, take the 1/4" 20 X 1/2" bolt and connect the Circuit Playground to the mount, making sure that the JST battery connection is facing up. You'll probably have to put some washers or other spacers in-between the head of the bolt and the mount. I used a convex brake washer since I've got a bunch of bike parts lying around, but just about any washer that fits will work.
When it's all attached, it should look something like this:
If you don't have a JST extension cable with a switch built-in, now might be a good time to make one. They're quite simple to make, simply cut and strip one side of any JST extension cable (or even a lipo itself, although this isn't ideal) and solder the two of the ends to a switch. Make sure to tin the wires and the switch contacts beforehand as this makes it much easier. In the example below, I cut both ends since I wanted something rather short.
This second design option mounts to the bike seat post and incorporates an integrated slide switch.
A tripod screw adapter is secured to the mount and features a threaded hole for screwing into.
A quarter twenty screw adapter is used to attach to the clear case.
The slide switch is press fitted into a built in holder behind the battery.
Move the switch to the center and then gently insert at an angle. The two metal sides will press fit between the wall on the case.
The case attaches into the tripod screw and easily connects to the JST adapter on the slide switch.
The battery is secured to the mount by sliding it into the pocket.
The slide switch can then plug into the battery.
The mount is designed to flex open so it can fit over the bike frame.
You can clip it right under the seat and slide up and down to adjust the position.
To secure the mount in place, insert an M5 screw and tightly fastened to a hex nut.