While the requirement for a manual fetch restriction is understandable, a manual file rename is a hassle as an extra required step. Unless there is some good reason that's not obvious to me, it would makes sense to change the ebuild to use the normal file name as downloaded from the TrueCrypt website.
I think the reason is because the original filename has spaces and due to how portages parses SRC_URI it then sees three different URIs instead of one with spaces.