What is the maximum amount you want to pay(I notice you only want something better than stock, nothing fancy)?
Do you want a direct drop in, or would you be able to create a 6x8" plate out of wood if needed? (5.25" and 6.5" would fit in a 6x8" plate, giving you more choices and still keep the speaker in stock location)
You should be able to get a pair of decent 5.25/6.5/5x7/6x8 speakers for $40-50.
