I know, it's nice and shiny and I would hate to see it corrode! If you bought a 100 amp shunt, it will put out about 5 watts at 100 amps; if you have a 500 amp it would dissipate 1 watt at the same current. However, the situation is kinda self limiting since you can't draw those kinds of current for very long on your rig. What we are looking for here is a current that lasts a while, say, charging a dead battery. Let say you get 40 amps for a while so that would mean you are dissipating .8 watts with a 100 amp unit and .16 watts with a 500.
Neither of these numbers should cause a problem inside a reasonably sized PVC box.