I think battery connectors are a big source of DIY headaches. It's not always economical in terms of space to use a battery sled. You can save a ton of space by using a more direct connection. In my case, I always solder my cells making them non-removable. It eliminates both the space consumption and the headache of connection quality. That can be a big deal with high demands on the battery. Of course, non-removable cells have their issues too, it's a pain to replace cells when they wear out.
In any case, copper is second only to silver in conductivity, but it also corrodes quickly. That tarnish build-up degrades connection quality. There's a couple things you can do. You can buff copper contacts then apply a layer of conductive grease to keep them from corroding. However, that becomes sort of a maintenance headache. The other option is to plate the contacts with a corrosion resistant metal like nickel. That's how the corrosion issue is handled normally, with a nickel or tin-nickel plate.
I have not actually looked into getting anything plated, but it should be pretty easy to find a shop that does it. Any shop that does chroming will be able to do that since the base plate for chrome is nickel.
Copper is a very soft metal so it's not going to make a very good spring base since it will just collapse and stay there. To get the spring effect, you pretty much have to use spring steel or a similar metal with high tensile strength. Unfortunately those tend to be crappy conductors. You need a heavy plate of copper then nickel to resolve that issue.
You could probably use an off the shelf spring and take it to a chroming shop for a good thick plate. I think the chroming shops actually start with a first layer of copper so they can do that too.
Magnets are poor conductors so it's actually better to avoid them. The Ne type magnets are especially poor condutors and those are more common because of their superior magnetism.