Tailgate mat 79 truck

[truckin 79]

Anyone have a good suggestion for a well fitting comfortable tailgate mat. I am a short guy 5'6 and have to climb up into truck bed all the time. I use truck for storage of parts I sell online. Putting my 50 year old knees on truck bed is painful. I have a thin rug type of material on there know but it is falling apart. I would like something felt lined with a thick rubber backing. All I see is rubber mats right now. What size is a standard 79 truck tailgate?