If the individual would be just as happy with orange as lemon flavour, adding chocolate and almond into the mix, check out the Golden Grand Marnier Cake in The Cake Bible where it’s baked in a 9-cup tube pan. Rose used to ship that cake to the University of Michigan for her daughter’s birthday. She added more Grand Marnier and airmailed it to France one time! That same cake, modified for round 6, 9 and 12” is also featured in RHC. I just made it for my own birthday, and it’s out of this world.
p.s. added by edit. Sorry, I know you were looking for a filling, but I couldn’t help raving about the GM cake. Thought it might be nice to know, if only for the future, that the GM is one cake that mails particularly well.