I use Ancor marine lead negative with the wing nut disconnect @ $5.50. A rubber cap is also available
Lead Battery Terminals Negative With 5/16" Post
West Marine also has tinned copper and gold plated terminals. Marine grade costs a few cents more than auto store parts but it has to be high quality to survive in extreme conditions
i never took apart one of those switches but anytime you have a switch or connector you introduce voltage drop. A switch is more prone to have a higher drop. I can only guess that the switch uses a springed connector instead of having a solid connection by using the wing nut. I drilled a hole in a wing for an easy recovery string as it's easy to drop it into the dark regions.
different strokes... etc