Look at the weather checking on that! Actually, it looks more like cracks from sitting flat for too long.
(An aside: I learned recently that boat builders - and probably other woodworkers - call cracks in lumber "checks". Probablly the realm where the word came from. Hey - I wonder if the meaning of "a checkered past" is connected.)
They're 13 x 5.0 size. I'll go look for inner tube options.