It's a good thing I am ahead of the game with this challenge since I lost most of the weekend playing 'Oxygen Not Included' instead of cracking on with my Primaris. Still most is not all and I got some more work done.
Here are the bases before...
I used cork and plastic skulls from the GW Box of Skulls for embellishment. I thought since I'm painting some Ultramarines that the rims be Goblin Green.
All that's left really is to get my marines mounted and the transfers applied.