Sorry, but that's why hotels call it a "day rate". Sometimes they will let you push the checkout time to 4-5 pm. Just explain that you have a later flight to catch and they may be understanding depending on how many rooms are booked that night.
I've found the Strathcona to be very accomodating. Not the classiest hotel in the city, but it does the job. Also, the Four Points Sheraton on the Lakeshore is a possibility, slightly more upscale.
