When I needed to replace some metal parts of the roof rack on my 1988 245 wagon, I was dismayed to discover that I could not screw the supports back into the rubber fittings that hold the rack into the roof.
The rubber had deteriorated to the point where the rack was unsafe at any speed - there was nothing holding it down!
The parts dept at my favorite Volvo dealer insisted that the rubber mounts were no longer available for the 245. I knew better. To order them you need the name and the part number. The part name is well-nut. The part number is 75211-3 . You need 12 to replace all the mountings. GM has a similar part with a different number, 528846. Google also liosts them under well nuts with several sources. Be sure to specify the well nuts for roof racks; others are used to mount cowls, etc.
If you must improvise, you can buy something similar in a hardware store, but they don't work as well as the o.e.m. part. To use the generic substitute, push out the threaded nut at the bottom of the wellnut and in its place insert a jacknut, The jacknut will expand and tighten the well nut agaist the inside of the roof as you tighten the screrws from above. Well nuts and jacknuts of various types and sizes are available on the Internet from American Trim & Upholstery - atrim.com. They have a wealth of other products for car restoration and repair.
With new well nuts all around my roof rack is snug and weather-tight again- good for another 17 years.