Thanks for the posts!
Actually it weighs 5.8 kg.
I read that working with HDPE is similar to working with wood, so I thought that I make the shape out of wood first, then I build it out of HDPE.

Here's the project made on SketchUp plus older designs:
Cattura.JPG
For my first robot I decided for a simpler design, mabye when it's time to rebuild it I will work on one of those other designs adding a weapon.

Also expect a video soonish.