I just picked up a 22Lt top box. Mounted it at the sellers place, what a pain. Seller was good and supplied tools and helped. I had to undo two carrier bolts and loosen the other two. The bracket slid in behind the rack but was extremely loose, it just rattled around in the hole. I ended up covering the ducktail with wide black insulation tape and taping the bracket to the rack. Once the top box was installed everything firmed up. For years I ran a cobbled up top box about 12Lt until it broke. I was unable to replace it with one the same size so ended up with a Shad 33Lt. It was too big for my needs and was affected by side winds on windy days.

I purchased the 22Lt off Gumtree it was in excellent condition and the seller had a key cut for it and threw in the duck tail rack. It was a very windy day and due to not having a top box mounted the wind had very little effect. On the way home with the 22Lt mounted it was the same the wind had no effect.

Next job is to make a top rack out of alloy or tube. I've come up with a couple of ideas make a pattern out of ply and have it cast and then finish it off. Have a slab of aluminium cast and mill it down. Or bend it up out of tube and weld it together. I'll have a talk to the brains trust and then decide.

Regards Martin.

Before you go to all that work, conside fitting a Givi topbox rack. I've no idea of the dimensions of your BMW topbox, but the rack  on my Givi 52 litre is on 260x180mm centres. The smaller topbox/pannier racks are (obviously) on smaller centres. As many Givi lids are slightly curved, the most you will have to do is to file/mill flat the foot of each mounting post. Done that to fit a later rack to an early (flat) topbox.

Thanks for the suggestion Dai, but I need something to keep me occupied in retirement. I am seriously leaning towards cast as once the pattern is made it is easy to replicate. I'll know more tomorrow when I talk to the brains trust.
Regards Martin