20-20mm box is way to over the top for a feather, you'd be better using 15-15 or mabye even 10-10, not only that but box section is heavy and you'll struggle to get it in weight - i shud no because its all i ever used to use.

i've got a 2 wheel drive box on wheels with a welded box chassis 1 x 12v sla battery and tbh its not go alot of box section in it and thats 8kg with only hdpe armour- you might get it in weight- you'd have to make it as compact as possible and go for hdpe armour as it'll be lighter