the sheth? is rubber and protects the wires and vacuum lines between the door and the body.
Rubber conduits is what you're looking for. Rubber The Right Way sells Steele Rubber Products' items for a discount. I'm sold out of good used, but here they are new. Since they're correct for a few other GM divisions, you may be able to find them for less through higher volume resellers. Please steer clear of OPG in your search. https://www.rubbertherightway.com/1966-cadillac-restoration-parts-power-37485-prd1.htm
thanks, Jeff