Waterproof A4/A3 Map Kneeboard - Sale was
This fits as an A4 on the leg and unfolds out to A3 as required, Held on the leg with Qty 2 x 1inch elastic webbing and plastic fasteners.
The internal measurements are about 11"x17" (Landscape)
Along the upper edge is a Velcro opening allowing you access to map pocket and to insert your map prior to flight.
The wallet is held in place by two thin adjustable straps under one leg