How to emerge UT2004 to a different folder/partition?

[Putrifier]

First time for me, trying to get Portage to behave as would be more comfortable for me. Sorry if the answer is really obvious, and I just don't get it.

Anyways, I don't have enough space left to install UT2004 (what a behemoth) on my Gentoo partition, and I was wondering whether I could somehow get Portage to install it on my data storage FAT32 partition, mounted as /mnt/win_d. I tried looking in the UT2004 ebuild, but I'm not sure I saw anything that could hint at how to change that functionality.

The Gentoo Portage manual names these important Portage-related folders, but I don't think any of those relates to where the installed files ultimately stay.

PORTDIR - Location of the Portage tree.
DISTDIR - Local cache of downloaded archives.
PKGDIR - Location of locally generated tbz2 packages.
PORTAGE_TMPDIR - Temporary space used for package compilation.
PORT_LOGDIR - Location used to store log files from individual merges. Logging is disabled if this variable is not set.
PORTDIR_OVERLAY - Location used to hold a local customized portage tree. Portage looks here before searching in PORTDIR.

Any help would be more than appreciated. Thank you in advance.

[Spooky Ghost]

Having a quick look and I think (IIRC) $D is the destination directory that portage builds the image in. I think you can override this or do D=$D/mnt/win.d before Ddir.