I don't think I have any fitting parts for Ultra Magnus. Yet.
I'd probably just make him a normal-sized Kreon, if/when I get parts. While I could use the combiner limbs to make him larger --like the Movie Devcon I made-- there really aren't any 'hands' for that.
On another note, decided to make Overlord a permanent part of my collection. Optimus Prime no longer has his helmet (or head), but I really didn't care for him anyways so it's all good!
Had to paint his legs (didn't want to take Hot Spot's from him), which ended up kinda sloppy. Too bad there aren't many cyan legs, much less outside combiner sets...
Also gave a new helmet to Kickback (same idea as Ransack's, just cut off the visor):
And it seems I forgot to post G2 Megatron in this thread: