[Dovecot] Setting IMAP root as user's home directory. How?

I'm trying to set up dovecot to use /home/username as IMAP root, but it keeps creating /home/username/mail. I need /home/username in order to migrate with the least changes from uw-imap, just by copying .mailboxlist without the INBOX line in it into .subscriptions

I tried each of these, one at a time:

default_mail_env = mbox:~/:INBOX=/var/mail/%u default_mail_env = mbox:%h/:INBOX=/var/mail/%u default_mail_env = mbox:/home/%u/:INBOX=/var/mail/%u

In each case, dovecot was stubornly creating /home/username/mail

Please CC me, I'm not on the mailing list. Thanks.