Hey mate, I've been pulled over twice now and nothing has been mentioned, my insurance didn't ask me anything when I said I had a steel tray I made myself, so I'm going to play ignorance and say nope. It's a bolt on modification so I don't believe so.
VicRoads admin staff who don't understand anything seemed to think it does require engineering, but they're not the ones paying for it. The real answer is no.
Ask, your insurance company and the RTA.
Do you need to tell the RTA if you change?
<I suppose I could get out the rego paper and see if it mentions tub or tray on mine>