No, don’t need a blue slip for an unregistered permit on a “complying vehicle” (this excludes crane trucks, harvesters etc) in NSW. The official form is here:
https://www.rms.nsw.gov.au/documents...eclaration.pdf
Q7 is the apparent arbiter, but it just handballs responsibility.
I don’t want to double handle the tub, it’s adding risk of incidental damage and wasted time. I have spare flaps, could make a ply sheet with them fitted, to span the rear wheels.