To be honest, it looks like you have enough cross bracing, and probably won't be carrying anything extremely heavy anyway, so....I would get a 4×8 sheet of 'Smartside' siding. Put finish side up ....it will last a LONG time as is! It's only about $25 at Home Depot. Now...to make it last for almost ever ....paint the bottom side before you put it on.... (This is some pretty good stuff: 50 year guarantee when used AS siding!)
Now....(time to get silly)... or get some window screen and spray it with $300 worth of FlexSeal.