You might want a label saying something like "DANGER High Voltage. DANGER Explosive Arc Flash Hazard. No user servicable parts inside, refer servicing to qualified electricians, proper Personal Protection Equipment required." Maybe add a description of what an arc blast is, and why it's dangerous: "A conductive object shorting the main feed will, with the busbars, be melted and vaporized, causing very hot material to be explosively sprayed at the worker, which can cause blindness and other severe injury."
To convince someone that "This ain't those little sparks you saw on your toy train set..." Part of the object here is to convince a homeowner that he ought not to mess with his panel.