This was the sign I was looking for.
What distinguishes this location from your typical Novus Ordo parish, until you go inside?
You could have this sign out in front of any random Novus Ordo parish.
As for the URL at the bottom, that can obviously be changed at any time to whatever the SSPX's new name is. It's such a small part at the bottom, it could easily be modified later.
I'll play devil's advocate here and propose that it would be a good thing for someone who didn't know better and expected the Novus Ordo to show up at the chapel and perhaps be converted by what they find. I have known that to be the case among several Trads I know. If they come in dressed in slacks and talking loud, then they will be in for a bit of a shock.
Also, I can see it as being a welcome opportunity to explain why there is no Mass differentiation on the sign because, for us, there is only one Mass and this is it. The other thing doesn't exist, at least for us. Among Trads, both SSPX and independent, you may also find disagreement as to what form the "TLM" is. If you go to St. Athanasius in Vienna, VA, the TLM is not the 1962 Mass, for instance, and it would not be an option for a visiting priest to say the J-XXIII Mass.
I would not use any signage advertising the Extraordinary Form, even if it might attract indulters. They already know the scoop. I say that it is best to keep the "form" out of it since there is only one that will bring one to salvation.
Now, if they came in and found the Novus Ordo, that would be the problem.