Record with rosbag from launch file

I want to use *rosbag record* in my launch file with the parameters linking to a ros package. Normally, you can get a package path in a launch file by using ${find mypackage}, but this does not work in this case, I presume because the *${find pkg}* command doesn't work in the *args* statement of the launch file (?). The error in the log shows that the rosbag node is trying to subscribe to topics named *${find* and *mypkg}/subdir*, which clearly is not what is intended. I definitely don't want to use an absolute path. Is there any way to specify the relative path here?