I pulled the cargo tiedowns from a couple of wrecked early-2000's Toyota Matrix cars. They are installed with a screw into a nut plate, with a penny washer under the screw head to ensure there was no chance of a pull-through. Very overkill solution in terms of strength. Folds flat to the mounting surface so they are not in the way when needed.
Price? The wrecking yard didn't charge me for them nor for the factory Toyota adjustable tiedown straps. All I had to do was grind a little spur of metal of the back of the mounting plate; Toyota uses it as an anti-rotation tab. If you planned ahead you could likely keep these little spurs and slot them into matching holes in your aircraft structure.