or sheets of thin black foam, very lightweight but cover up all the metalwork . sharp edges
This - for about £20 and a few Stanley blades.
Don't cage it unless you have deep enough pockets for buckets and harnesses, otherwise you're going to smash your head open on the steel bars in your interior if you have an off.
Side-impact protection bars in the G3 are worth losing if you're weight-saving, as they're knocking on 10kg alone.
You should relocate the battery and washer bottle to the rear of the car to try and help with the weight distribution.
I hope you've re-secured the controls from the dash properly - last thing you want is them coming loose and firing about the cabin. If it were me I'd put the dash back in, stripped of any soundproofing you can. They don't weigh very much and it's useful having mounts for your switches etc all there, ready made.