Making aluminium conductive for an RF shield enclosure
If I need to ensure good contact to a housing, regardless of the metal it is made of, I use EMI-fingers or collars where possible.

You can get them made to almost any spec, from cheap tin plated to 3&mu;m paladium-gold finish.

Here are a few examples:
Weird little spring-finger that might be slotted in
A type of EMI-Gasket/collar
And another EMI-Gasket
A wrap around EMI-Finger, that gets mounted on the lower flange and then presses down onto the board after wrapping around it

Those beat out any, and I mean any solid metal object with any kind of finish, because rails and other such do not apply any force. Try it at home: Get a nice clean metal surface (copper, steel, whatever), apply a voltage and then measure that voltage, preferably with a light load, like a 1k resistor, while softly laying a contact on that plate. Then apply a tiny bit of pressure. Note the difference in the measurement accuracy.

Contact resistance is directly proportional to the amount of pressure applied.



That said, I agree with Andy's comment that if you need good contact to the housing to eliminate noise that comes from your own PCB! there's something very wrong in the design.

It sounds more like you require the casing to be a ground plane, than an actual shield.

Good use of ground planes and stitching vias and though through layout of noisy and noise sensitive areas should take care of all of that, even if the board is dangling in free space. The reason to need reliable contact between housing and board ground is noise from the outside, not noise from the inside. The latter being one that requires much fewer connection points. (usually one global connect and one accompanying any sensitive signal, through coaxial wire for example)