Tar roof References needed

This Summer we're planning to replace the roof on a Rio Grande short caboose (0579) during the Friends work sessions. Even though it's obvious this needs to be a tar-covered canvas roof, I am looking for any kind of specfic references, ideas, or considerations on what needs to be done. The existing roof can't be used as an example. We want to do this historically correct, but most importantly, we want it to last a long time !
Most cars on the C&TS are either metal or rolled asphalt; I'm not sure the Friends have ever done a complete replacement.
I found a wee bit of information in an old L&RP magazine, but so far I haven't seen any other documentation on this type of work. Does anyone recall seeing any information on this subject ?