Take from my image what you will for my idea.
This, when welded together, will provide a solid/removable set of castor wheels for both L & R sides of the box blade.
No need to fuss with getting anything "just so".
When you're done using the BB scraper, attach the wheel-sets with pins & clips. Set the BB Scraper down, roll away.
Push the whole works easily to where needed.
This configuration is figured for a 48" (max 300lb based on 3" steel wheeled castors) BB Scraper.
*modify as you see fit. This is a rough draft*
It can be modified for any size and weight.