Thanks to everybody who responded to my question about a flat coax jumper.
I finally found it. It's the COMET CTC-50 WINDOW GAP JUMPER. The jumper is rated at 100 watts HF, dropping to 10 watts at 900 - 1296 MHz FM. I'll be in contact with NCG for more information, especially dimensions and bending/compression limitations.
I may finally get on HF without having to bore through the exterior walls and insulation!