You have to create a .conf file. Otherwise you are stuck with the defaults and have to use the mount commands, etc. to configure the environment.
Do a man dosbox to get information and look at the following page:
http://dosbox.sourceforge.net/wiki/
Look at the various resources there on how to create a .conf file.
Use the syntax found on the man page for DOSbox on how to make it load a particular conf file (I recommend one for each program you run, specific to that program's requirements).