[stephen low]

Sambo

If there is enough meat in the chassis you could drill and tap and attach directly. I've done this for fittings in the engine bay.

Another good trick I was put onto are aircraft anchor nuts. Essentially a captive nut held in a frame, with the frame riveted into place to hold the nut where you want it.

Come in all different thread sizes and even plain, nyloc or metal lock threading. means you can unbolt bits very quickly once all assembled.

Just another thought!