Paul, I also have the Zega panniers, although I have 2 x 41 Litre.
I noticed that they looked fairly pristine on the top, meaning that you haven't used the top hooks on the lids much, if at all.
In Australia there is a small firm that is selling heaps of straps that are called "Andy Strapz". His name is Andy and he makes straps for motorbikes.
His web site is
http://andystrapz.com/
When there, look up Flat Strapz and that is what I'm talking about.
I have 2 of the 1.5 metre straps and they are perfect for attaching a sleeping bag to the pannier lid. The straps are 50mm (2") wide and are all elastic, so they stretch. The attachment is done by using REAL Velcro and I have to say they stick like cow dung to a blanket. Even when soaking wet they work perfectly.
On top of each pannier lid I have attached 3 x strips of thin rubber that are 50mm wide, lengthways between the hooks of each lid. These strips are designed for wet areas around Saunas and the like and come in a roll with sticky stuff on one side. By having these pieces of rubber, the sleeping bags which are covered by plastic bags will not move and are completely waterproof.
I also think that you have done an extremely good job of manufacturing or acquiring a terrific luggage rack.
Is it difficult to gain access to the tool kit?
Why didn't you remove the upright on the rack, thereby allowing you to have one clear flat luggage area?
The piggyback Strapz on Andy's website are really useful, and the pictures show just what we have all been doing but with great difficulty up until now.
Mick.