Vigorport Luggage Straps, Luggage Connector Straps for Suitcases Add A Bag and Connect Your 2 Suitcases Together Adjustable Attachment Accessories Belts (1'' Width S Size)

The 35L duffel itself was a bit unwieldy to be on top of a roller\spinner suit case, imo. I can only imagine the 50L being more of a hassle, and slipping off readily. For my 35L, I looped the carry handles through the roller bag's handle, and then used a luggage strap to secure it. https://www.amazon.com/dp/B07SKZ9WWS https://www.amazon.com/dp/B0C84VWVX1 Since the bags are large, if you pack them heavily, the suitcase can tip over.