Pic shows: New tiny bag size allowed on Ryanair planes for free. Checking sizer at all the gates to stop passengers see here at Stansted Airport. P Stock Photo - Alamy
Goruck GR1 21L in Ryanair small cabin sizer (would therefore fit Easyjet, Wizzair and others as well) : r/onebag
Ryanair's Bag Policy – Ryanair Sandbox
Ryanair Cabin Bag Under Seat Baggage 40x25x20cm Luggage Carry On Travel Holdall | eBay
The ultimate guide to Ryanair's cabin baggage policy and fees