Use differential pairs, 100Ω, no ground plane. May not actually be able to achieve that within acceptable PCB limits (trace spacing), so just get it as close as practicable and keep the routed lengths short so the discontinuity doesn't affect it too much.
Note that you can do edge-coupled differential on a single layer, buried edge-coupled (on an inner layer), or broadside coupled stripline (on top of each other, different layers).
Tim