I baked my pie at too low of a temperature. How can I save it?

It's not unusual for a custard pie to have the edges brown and set long before the center. In fact, if you bake it until the center is entirely set, it will be overbaked and likely to crack. If the portion that seems liquid is about a quarter or half-dollar in size, and only slightly jiggles, your pie is done. It will continue to cook after removing it from the oven, and the center will set then. If it really seems more is liquid than that, you can try placing a foil sheet over the top to shield it some. I'd spray the side of the foil that will face the filling with a non-stick spray to help prevent it sticking to the filling. If you form the foil over a serving bowl, you could form a dome shaped shield to use instead of a flat sheet. Either way, you are not wanting a fully set pie filling. You want that bit of jiggle in the center when you take it out. I don't think your mistake is terminal either. Custard really prefers to be baked gently anyway, though your first try was a little more gentle than expected.