My husband traveled with friends to the Sierra Nevada World Music Festival in California. Before he left, he asked me to make him a nylon pillowcase. He camped for 4 nights and wanted something that was water and stain resistant. This was an extremely easy sewing project. I didn't measure a thing. I cut the nylon larger than the pillow, so that I didn't have to worry about the pillow not fitting. I also intentionally made it roomier so that he could tuck other things such as his wallet into the pillowcase for extra protection.
My husband wanted velcro at the top. I bought velcro that was sticky on the underside. This made placing the velcro on the nylon extremely easy and then I enforced it by sewing around the edges. The entire project took about an hour or so. It's not the most beautiful thing, but it's practical and it worked really well on his trip.