Hi

I would use a USB connector and cable. These contain two (possibly
three) twisted pair with a shield around them. They are controlled to
93 ohms differential impedance, and do not have significant reflections
below 1 Ghz when terminated properly. They are cheap and plentiful,
and available in a variety of sizes for PCB mount.

Mouser sells a good assortment--I usually go for the 'B' size for PCBs.
I use these for an RF differential probe I designed, with good
results.

There are also lines of shielded telephone connectors and cable
available. For a 1 Mhz clock these would do well, and do not have as
low an impedance, which might be easier for your 485 drivers to work
into.

Regards,
Glenn