So the glue definitely made it stiffer. Then the 1/2" walls started to curl and bend over every time I hit it with a ball. So then I beefed it up with 1" tall steel walls, and that made the opening just a little too tight (ball would either get stuck, or bounce back at the flipper). After bending the opening back, it started to tear a little bit. I tossed the ball at the ramp in frustration.
So what did I learn from this?
1. If you've got an aggressive shot where the ball has to make a sharp turn, make SURE your walls are high enough
2. Even stiff walls that are very thin need reinforcing (or just make it out of thicker cardboard to begin with, perhaps a double thin layer
3. I really don't think I can reliably have a side loop shot, PERIOD! Even if you can occasionally hit it just right (which I did get maybe 3 out of 10 shots), while it might be rewarding it's going to be very frustrating and NOT fun, and the ramp is probably going to get beat to hell during all the missed shots. So I think my solution (since early prototypes worked great this way) is to implement this ramp so it can be shot up and down, not side to side. If you watch people play Gottlieb Gold wings, even that ramp is somewhat hard to make, and it's as vertical as can be (and has tons of guides to keep it on track).
So that's my next goal, figure out where I'm sticking this ramp (after I rebuild it one more time).