What's the best way to anchor a porta potty on a windy site?

A rented standard unit on open Montana ground usually needs more than the short stakes that come with a normal delivery, and you should ask the rental company before you add anything of your own.

What works for us on windy sites is a mix the supplier already knows how to install: longer ground stakes or screw anchors tied to the unit corners with soft straps (not chain biting into the plastic), plus ballast. Water barrels, sandbags, or concrete blocks on the skid pads keep the base from walking without drilling holes in the shell. Never bolt or screw into the walls or roof yourself. That is the fastest way to crack a panel and get billed for damage.

Also check the pad. If the unit rocks an inch after a gust, part of that may be soft grass under one corner. Relevel on firm ground or a sheet of plywood so the stakes pull even. Point the door away from the prevailing wind so a gust does not catch it like a sail when someone opens it.

Call the company, describe the wind, and ask for their wind kit or a ballast option on the next service visit. If they approve your own sandbags or barrels, great. If they want to supply the gear, let them. That keeps the rental agreement clean and the unit still serviceable when the vacuum truck shows up.