FWIW we have a nice repro of the shift boot used on most 60's Fords as well as the Cobras and Tigers. It is done with a supple rubber with UV protectant embedded. Some boots are so stiff they "pull" on the lever and can cause the lever to pull the trans out of gear.
https://www.timemachineauto.com/m38-...helby-csx.html