What is the proper way to perform work in an attic? (UPDATED)
Honestly, I would get some sheets of plywood (is OSB stiff enough?) and rip em down to 2x8 or other appropriate size, making a compromise between spanning enough joists and fitting in your car.  Float them around as needed to do your work.  "Just step on the joists" has a fairly high penalty for error, at the least, refinishing a ceiling.