What You Will Need
An alternative option for the LEDs if you don't feel like doing any soldering would be to get a strip with a power supply (Amazon search example: red led strip with power supply)
The first step is to fill Han with hot glue. The translucent glue makes a great diffusor for the LEDs.
You will need to do it in a few steps, waiting for the glue to set in between each, in order to build up a nice thick body, without the glue running all over the place.
If the glue ends up where you don't want it, you can easily pull it out, or if it hasn't set yet, use a wet finger to remold it.