Panels on the side, especially if screwed/bolted, are going to be pretty good for stability and overall robustness. An alternative to that would be metal braces, corners, etc. Specifically cross bracing.

I would suggest to look at existing designs for how to reinforce the angles. Also, there caveats specific to wood designs (wood fibres direction, proportions, shapes, wood type, quality, density, etc). You might want to ask on a woodworking specific forum.