How about a narrower box blade, maybe 4' wide, & then affix some additional weight to it at some point, if needed?
Land Pride's 4' wide BB1548 weighs 382 lbs
Land Pride's 4.5' wide BB1554 weighs 402 lbs
Either one would be a good start.
At least you'd have a useful, albeit narrow, box blade, instead of just carrying around dead weight that has no other purpose/ usefulness.
Edit: Land Pride, not Woods ... Sorry
Edit 2: Wood's GB48 weighs 428 lbs