A Scope Filler for the Nerf Motorblitz, the scope area is just to far back to see thur.  The Filler allows for a cheap flash light and a volt meter to be installed there.  You should be able to unscrew the flash light to change the batteries without taking the blaster apart.

Things your need:

Volt meter, Cheap Light flash, small switch, wires 

A pic provided shows where the volt meter, flash light, and main body goes, here is the rest:

1. Solder all connection together and glue the switch in place
2. Take the light. apart and remove the rise area at the end on the large section. 
3. After you place the light in you will need to turn it until it lock in with the tab.
4. Put the light trigger in the body from the top hole.
5. Next slide the pin in place thru to lock the trigger in place.
6. As the Volt meter is placed, place the meter trigger for the switch into the hole at the end of top.
7. Place the meter pin in place under the meter