[Zeke]

I would not use a plywood hollow 'beam' outside. Wood is wood and nature eventually sees that it turns into fertilizer for young trees. That's not the case with all the glue in plywood but nature does a good job of rebuking the glue too. Nature hates that glue and tells you so.

Real wood for real structures outside. Treat it with all the poison you can for more life and kill something else. You can be somewhat nature sympathetic and use natural oils.

I never use plywood outside, even T1-11 if I can help it.