Bottom half of case. Clear out battery hold-down holes with a #50 drill bit and pin vise. Carefully tap the hole with 2-56 tap. 

Top half of case. Very carefully drill the 7 thick round mounting posts using a #42 drill bit and pin vise. Go slowly and clear out waste a couple of times while drilling holes. Tap these holes with a 4-40 tap. Tap the holes even more carefully than drilling. Keep the tap clean. Do not drive the tap too deep. The posts will snap off. 

[Warning: This step has risks. The battery is very powerful. Do not let the two battery leads touch each other.] See Step 11 for a picture. Cut the connector off the battery keeping most of the wire attached to the battery. Cut the JST-PH battery extension cable so that the female connector (the connector that has no exposed metal) has about 3" of wire on it. Strip the battery wires. Strip the wires on the female connector.  Mount a piece of heat shrink tubing on one red wire and one black wire, if you plan to insulate each connection that way (see below). Twist exposed leads of black wire to black wire and red wire to red wire. Solder the two black wires together. Solder the two red wires together. Now the battery has a new, longer cable with the correct connector. Insulate the connections. This can be done with heat shrink tubing if you have very small tubing. However, I use the J-B KwikWeld epoxy. Coat each connection with the epoxy. Be sure no wire is exposed. Do not handle wires for 20 minutes so the epoxy can set.  

Bottom half of case.  Mount battery using 2-56 screws 3/8" long. Use the printed battery strap. See photos for positioning. Keep the strap a loose fit because these LiPo batteries expand slightly after a few charges. 

Use a small screwdriver to split open the plastic case of the keyfob near the key ring clip. Remove the circuit board and discard everything else, including the batteries. Transparent tape holds the two metal pushbutton in place. Remove the tape and discard the tape and pushbuttons. 

Place a drop of solder on the center of each pushbutton contact on the keyfob circuit board (labeled K1 and K2). Each contact also has a metal ring. Put a drop of solder on one of the metal rings. Take the female Micro JST 1.25 mm 3-pin connector. Solder the red wire to the K1 center contact. Solder the yellow wire to the K2 center contact. Solder the black wire to one of the metal rings. 

Top half of case. Study how the keyfob will be positioned into the case. See photo in Step 11. The K1 and K2 solder connections will be pressed gently against the inside top of the case. A tiny LED at the top of the keyfob (top of photo in Step 6) should line up with a hole in the case. Hold the keyfob into position and then flip the case over to get a good idea of the LED alignment. You should see the LED through the hole in the outside of the case. Look at the photographs to see how the Micro JST connector wraps around the keyfob. 

Use a thin piece of duct tape or electrical tape to hold the keyfob in position for gluing. Epoxy the top, sides or bottom of the keyfob. Not much glue is needed and the circuitry would rather not have too much epoxy on it. Once the epoxy cures, carefully remove the tape and and epoxy one or two more spots. You can also put some epoxy on the Micro JST wires as they emerge from the back to the keyfob circuit board.

Solder 3 wires to the LiPo charger board. Each wire should be about 5" long. Solder two to the middle two GND connections and the third wire to the BAT connection. The +5 v connection is not used. In the photograph I have used green wires for the ground and orange for the battery. Black wires are more commonly used for ground. 

Bottom half of case. Check the fit of the tactile switches on the two switch mounts. See photo in Step 13. When the switches are properly in position, the switch leads should face the front and back of the mount. (The sides of the switch mount are raised.) Remove the switches, put some epoxy onto each of the two switch mounts and then press the switches into place. Try not to glue the front switch leads to the mount. Leave the switches undisturbed for an hour to let the epoxy cure.

Top half of case. Work the slide switch into it's recessed position inside the case. Hold the slide switch in position and glue each flange inside the case. Keep the switch in place until the glue cures enough to hold the position, then give it at least an hour before testing the switch. Picture shows assembly to this point.