[masraum]

Prop rod(s) like an old fashioned hood support, seem like the easiest solution.
I feel like one on both sides would be good, but maybe you could add a hood strut to the other side. My main concern with the hood support would be if the portion that screws into the wood would be sturdy enough. Maybe some larger inserts that it screwed into would be required.
The support could be hinged to fold up along the bottom of the box when not in use. (it's going to need to be longer than the box is tall to put it comfortably low).

closed frontal view with the support hanging down top lefg. open angled view in the bottom right