I think the default is set in the package itself and can't be easily changed. However, when you create a new user, the files in /etc/skel get copied to the new user's home directory. You could create a new user, set it up as you want the default, and copy all that user's dotfiles into /etc/skel.

You mentioned using a guest session, though. I'm not sure if my method will work in guest sessions. Try it and see.

If it doesn't, here's another option: Create a guest user. Then put something like the following in ~/.logout:

shopt -s dotglob

rm -rf ~/* 

cp -a /etc/skel/* $HOME

This should cause the user to be reset every logout.