If you use a trunk bag (or rackpack or racktop) over your rear panniers like us, you will find that it sometimes needs to be fastened down with more than the built in clips. We have found toe straps (from pedals) to be the most secure way of keeping our bag sitting tight – especially on bumpy roads.
Many others use octopus (occy) straps to secure their load, but we've found toe straps to be smaller, lighter and make it easier and quicker to take the bag off.
We connect the toe strap just under the top tube / seat tube junction and to a handle on our Ortlieb Rackpack 49 bag. You can then tuck the excess part of the strap through the gap.
Once it is properly secure, there is not much which is going to allow the rackpack to slip backwards!
I’ve always just used the handle of the rackbag itself – just loop it over the saddle and let it rest against the seatpost. As long as you also use the side straps to attach to the panniers, it’s reasonably stable. It can sometimes flop forward towards the seatpost (well it can in your set up too), but never falls off the back. Also, I’ve got the exact same brooks saddle-cane creek seatpost-ortlieb 49L rack bag combination!
Great idea – we’ll have to give this a go!