Adapters work decent if they're secured well. Fitting a 6.5 speaker in a 6x8 spot is nothing. You're talking a little less than a 1/4" on each side of it, and with the adapter, you're lifting it out of the hole, so the taper of the basket more than makes up for it. Some adapters actually space the speaker out an inch or so, making original speaker size nearly irrelevant.
Personally, I recommend making your own out of wood. Your sound will improve over the flimsy plastic ones.