Oh I didn't see this line in src_unpack()

So the three solutions.
1.
define SRC_URI="${URI_BASE}${URI_BASE_NAME}.tgz -> ${P}.tgz"
of course this will break on every update from them
2. 
Ask some gentoo dev to host a versioned tarball for you on gentoo infrastructure and download it from there.
3.
make the ebuild fetch restricted and ask the user to download the tarball and rename. This will break as well on updates.