Making a deer stand

Anyone have any suggestions on material for side panels?

plywood works

Or fiberglass

Plywood probably the most chosen, cheap and easy to build with, metal is another option like "R" panel.

Good exterior grade plywood, good paint primer all over double coat the edges, good exterior paint, supports on frame to stop warping. Mine have lasted for 8-10 yr with just re painting as needed.

One place to always paint is the downward edge of wood (bottom edge), water will pool to there and drip, likely the last place that will dry.