You can choose what side the power connector is on before you fix the base.   If you have a bike computer on the left side of your handle bar then the light will probably be mounted on the right side.   You will probably want the power connector on the left side of the lamp as viewed from the back end of the lamp.   

If the bike computer is on the right side of the handle bar then it would be best to have the power connector on the right hand side. 

To start the operations here you should already have the electrical components soldered and fixed to the lamp.  See this instruction page for making the electrical connections.

Cateye space plate 533-8730, m4 x 16 mm pan head screw.

Place pan head screw in center hole.

Tighten down a M4 nut.

Place a large washer over the  nut.

Screw on and tighten the second M4 nut.

Place second washer over the nut.

Optional:   Loctite thread locker.  

With optional Loctite.

Assemble and tighten the third M4 nut. 

Cut a small piece baking paper and place it over the
slot with the spring in the Cateye mount.  
This is how it looks when it is done.

Cut off small equal pieces of Milliput

Cut equal piece of Milliput.

I recommend using rubber gloves when mixing.
Mix the Milliput thoroughly together.  
It should be all black when mixed properly

Decide what side you want the power connector on.
Orient the lamp and mark the cooling rib in line
with the two lamp pins.

Press Milliput into the center three cooling ribs. 
Note how the Milliput does not go to the
end of the cooling ribs.
The power connector is on the right, so this lamp would be
mounted left hand on the handle bar. 
Make an indentation in the center cooling rib,
close to the white plastic lamp housing.
This will help give you a position for assembling the base.

Place material on the base as shown in the picture.  
Leave the back end free for later forming. 

Press the mount and the light together on the Milliput. 
The screw end should be in the middle of the three
cooling ribs filled with material. 
The tip of the screw should be close to the
end of the aluminum housing.

Make sure the mount is in-line with the center of the  lamp
perpendicular and that the mount is not rotated relative
to the center of the lamp.  

Check to make sure the base is aligned with the housing.

Set the lamp in a safe place and let the
Milliput harden for 2-3 hours.  Monitor the position
of the housing to the base so that it does
not droop over time. 

Start to apply material the base. Fill in two addition cooling ribs
with material right and left of the ones filled for the mount. 

The base is filled out here.   
Apply material under the switch and electrical connector,
and in the void on top of the lamp. 

Apply material to blend the base with the connector area.

Start to smooth out and fine form the base.
You can smooth the finish with small amounts of water. 
You could stop here and let the material harden.

Apply material to fill out the connector and switch. 
Finish the area of the connector and switch.  

Make sure material covers the white plastic
connector housing of the lamp.  
This will help to seal the lamp
You can smooth the finish with small amounts of water.  
You could stop here and let the material harden.

The Milliput surface can be finished with wet
sanding and buffing.