I do like Marien's proposal. I myself have the very embodiment of a cheap box robot, but it is well under 10kg. I do think, however that here are some robots without active weapons that are frankly more exciting than a robot with a very basic active weapon, for example Pillow Torque. I would, as an onlooker, much rather see Pillow Torque careering round the arena hitting things and making a loud noise than watching a robot slowly lifting its opponent and dumping it in the pit without all the excitement of high speed collisions. Unfortunately, I am guilty of building a lifter. This type of weapon is good for the owner of the robot, but frankly not so interesting from the audience's point of view.
Just some ideas,