So, what kinds of stall Pokemon have you had success with?
Forretress, Rotom-a, Swampert, Blissey, Gyarados, Zapdos, Roserade, Celebi, Hippowdon, Starmie, Jirachi, and Dragonite if you consider it a stall Pokemon. Pretty much most of the standard mons that you see in stall teams.
Are there any lower tier walls that you find highly effective in DPP OU?
Hitmontop's an alright Rapid Spinner, and Nidoqueen seemed to be good way back, but I haven't used it in the current metagame so I can't say much about it. Spiritomb is also cool because it can take out Gengar and Starmie through Pursuit.
Do you have any custom stall sets that you feel really shine?
Just some EV spreads that aren't that much different from normal ones.
How does your team deal with wallbreakers such as Infernape?
Gyarados and Starmie usually handle it fine. Specially defensive Hippowdon can tank a Grass Knot and take Infernape down with Earthquake.
Are there any walls that you find disappointing?
Dusknoir, definitely. Spiritomb and Rotom-a outclasses it.
What about walls that you just can't seem to make a stall team without?
Rotom-a! I always use it.