How to get -RC2 is tarball name?

[Gary Aitken]

I'm trying to fetch:
   https://download.gimp.org/mirror/pub/gimp/v2.10/gimp-2.10.0-RC2.tar.bz2
Both the RC2 and the bz2 are giving me problems.
I've tried every combination I can think of and can't seem to get the
right thing.  In order to get the -RC2 I had to resort to:

PORTNAME?=  gimp-devel-app
DISTVERSION=    2.10.0
GIMPVERSION=    RC2
DISTNAME=    gimp-${DISTVERSION}-${GIMPVERSION}
MASTER_SITES=   https://download.gimp.org/mirror/pub/gimp/v2.10/

but that asks for .../v2.10/gimp-2.10.0-RC2.tar.gz
tacking the .tar.bz2 on the end of DISTNAME doesn't help.

I know this totally ignores all the other directives about
   MASTER_SITES= GIMP
and recommendations for PORT/DIST NAME and VERSION, but I can't get
any other combination to get even this far.

Is there a way to get the portname "gimp-devel-app" truncated to "gimp"
with some operator X in ${PORTNAME:X}?  I didn't see them described in
the handbook.

Is there a variable I can override for the tar.bz2 part?

Thanks,
Gary