Okay. I have come up with alternate layouts of the 10 Cob and 16 Cob. I haven't tested these, but you can do so yourself.
Key:
Cob = Cob Cannon
G = Gloom-shroom
W = Winter Melon
T = Twin Sunflower
U = Umbrella Leaf
E = Empty Space
10 Cob (immune to Ambush Zombies and Dolphin Riders, vulnerable to Bungee Zombies, Imps when timing gets off, and Catapult Zombies)
G G W T Cob
Cob W G Cob G
G Cob Cob T E G
G Cob Cob T E G
Cob W G Cob G
G G W T Cob
Note the Gloom-shrooms in the back that make the very back cobs immune to Diggers. Also, Imps shouldn't chew on those cobs when your timing isn't off.
16 Cob (immune to Catapult Zombies and less vulnerable to Diggers, more vulnerable to general zombies)
T G W Cob Cob
U G W Cob Cob
Cob Cob Cob Cob T
Cob Cob Cob Cob T
U G W Cob Cob
T G W Cob Cob
Note the weakness of this build: zombies get to your cobs more quickly. Imps may also be able to reach those back land Cob Cannons, but I don't think they can eat those squares unless they're Pumpkined.
What do you think, guys? Please point out any weaknesses I may have forgotten about, if you notice them. (Yes, I know both are vulnerable to Jack-in-the-box Zombies. Just bring replacement Gloom-shrooms or Cob Cannons.) I'll upload screenshots (yes, they will technically be hacked, but only for demonstration purposes) of these soon, probably Friday afternoon.