Hi Svenn
Window channel looks something like this same stuff that was used cars 40s-60s for the side windows to slide up and down in.
Measure your channel width and check with automotive glass shop they should be able to set you up with 5 or 6 foot long stock. It comes in different widths so be sure to measure your channel. Here is an on line source for it but I'm not sure of the size. Main page
http://www.steelerubber.com/ try this search year 1945 Chevrolet & GMC Truck and Van part name is Side Window Run Channel.
I'll measure the thickness of the plates today
Cheers Phil