Re: Flexible all-weather USB3 cable?
Look for TPE (thermoplastic elastomer) or PU or PUR insulated (jacketed) cables, which is are industrial cable insulations rated to -40C and colder.
PVC is typically consumer grade and gets brittle in the cold, as observed
I have made DC power cables from silicone insulated wire of the type used in model airplanes, cars etc, but it is easily nicked and cut, so I protect it with braided sleeving or other. Silicone wire insulation also doesn't seem to be UV protected so is not recommended (by me at least) for a permanent installation. I saw a friend's install for his portable RV solar panels go bad in one summer season when the insulation bleached and cracked.
L-Com has high flex TPE insulated USB 3 cables in this series but they are all short: https://www.l-com.com/usb-usb-30-high-flex-type-a-male-to-type-b-male-cable
They also have PUR insulated high flex USB 2 cables in this series: https://www.l-com.com/usb-high-flex-usb-20-cables-polyurethane-jacket-type-a-b-connectors
And high flex outdoor rated TPE Ethernet in these series: https://www.l-com.com/ethernet-cat5e-rj45-tpe-utp-continuous-flex-outdoor-industrial-ethernet-poe-patch-cords
Be warned if you are out of the US that L-Com ships via UPS and UPS did line by line brokerage charges that ticked me off. $67 in brokerage fees alone for a US$170 order of 5 line items. Grrr.
Another alternative is Lindy Cromo USB cables available via Amazon. They are a fancy audio cable maker from the UK and their Cromo line seems pretty nice. Officially rated -20C to 50C operating range. eg: https://www.lindy.co.uk/cables-adapters-c1/usb-c449/0-5m-usb-3-2-type-a-to-b-cable-5gbps-cromo-line-p11357
Personally I have sets of USB 2 and 3 Lindy Cromo, as well as the aforementioned L-Com PUR and TPE insulated USB2 and Ethernet cables, but I have only used them down to about -12C before the skies tanked here for the season (Calgary AB).
As for making your own, I have heard of people doing it but not tried it myself. Connectors can be purchased from the likes of DigiKey. Ensure you get the appropriately rated cable, too, and have good soldering skills as you will be soldering fine wires to small pins in close proximity and you cannot do a sloppy rats nest of bends at the connector as it could impact the signals.
All the best