I definitely see the value in a tailbag that has top access, since my Kriega packs don't. 

This Saddlemen Sport Tunnel Tail Bag looks like a cool tailbag if you have a tiny rear seat like on a Speed Triple, Daytona 675 or similar. A customer came in recently and purchased one for her Daytona 675. It looks like the straps work under the seat so if you don't have any mount points below, this should work well.

Another bag I recommend for top access is the Cortech Super 2.0 Tail Bag (available in 14L or 24L), but you do need external mount points to attach this to your seat. Cortech also offers an optional rain cover if you choose the larger 24L option.