I have done this on SLIGHTLY smaller boards with the largest toaster oven I could find, and a thermocouple ramp and soak controller from eBay. This oven is roughly 228 x 300 mm (9 x 12") and I have done single boards close to that size. The trick is to use micro-size thermocouple wire and pike the junction into a plated through hole in the board. That way, it is controlling actual temperature of the board. The ramp and soak controller operates a solid state relay in series with the oven's thermostat, and allows you to program temperature ramps over time and temperature holds. I've done over 2000 boards with this setup in the last 14 years.
Jon