Well, how thick is it in the official 7x10 pan? Probably thin enough that the closest edge is nearly always the top or bottom for any interior point? Then what you want to do is pick a substitute pan to get as close as possible the same thickness, which means as close as possible the same area, which means the 8x8 is much closer than the 5x10.

Making it much thicker would be likely to change how the middle came out quite a lot, usually.