Stretch fit with velcro ties.  These seat covers are just amazing.

With an open weave design, this allows water to pass through the cover and drain away, thus you don't get a sopping wet backside.

When the suns's shining on the seat, you can jump right on and won't get that burning sensation. Even better when riding down the track or road, air pass's through under you providing some additional cooling.

As an additional benefit you will also receive some extra padding and comfort.