Hardware for Headboard

My client, a small retailer, has asked me to make a headboard for a display in his store. He currently is displaying a mattress that simply sits on one of those basic metal bed frames that are everywhere. My question is, is there a special type of hardware that I should purchase that will allow the frame to bolt onto the frame? I am concerned that if I simply use a nut / bolt/ washer to fasten it that it won’t be stable enough if someone sits on the mattress and leans back on the headboard.

Any thoughts on hardward, or techniques, to fasten this headboard such that it stays solidly affixed, and looks professionally done will be appreciated.

-- Bob Card