Depends on type of walker you have. You could fashion one with fabric and small piece of wood if walkers has wheels.
If you can't find plans, it looks like there are some reasonably affordable options on Amazon that say they are universal and fit all (or most) walkers!
Walmart and Amazon sell this one that fits on top of the walker- https://www.walmart.com/ip/Walker-Tray-with-Non-Slip-Grip-Mat-Red/47484948?
If you want a DIY this is a wood one to make- https://woodworkingweb.com/creations/1191-walker-tray