If a physical sizer is used then I should be okay.

I was also contemplating putting everything in a slightly too large duffel bag. I could temporarily roll up one side to get to to squish into a sizer.

Air Asia allos one carry on sized 56 x 36 x 23 cm plus one handbag, https://www.klia2.info/airlines/airasia-baggage-info/

My duffel is 60 x 32 x 28 cm.

Now I'm not sure if the duffel is safer or a backpack plus straps.

The backpack and straps would be less bulk. The duffel would more strongly represent one "thing."

What do you guys think is safer?

Personally I would prefer the backpack and straps for easier travel but I'll use what works.