I tried to access NocanMessage member "dlen" in my program because that is what I saw in github in nocan_ll.h. The compiler said 'no member "dlen" in struct NocanMessage'. I was puzzled for a while thinking there was some strange visibility issue. I finally tried to access data_len, because this seemed to fit your pattern of naming. The compiler liked it. I'm thinking that the zip of your library does not match the checked in source code. Was data_len ever called dlen?
I'm still figuring out "best practices" and coding style on Arduino. Normally, I'd prefer accessors for internal data fields. I suppose it is a waste of memory on a small processor.