Thanks for all the info! From what you said, my understanding is that you got the hardware from Caribou & attached a Pelican case - which sounds ideal due to the huge choice of cases and inserts they offer.
Actually what I did, after trying the foam method, was getting a smaller a case to fit inside the BMW top case. In this manner I'm just using the BMW top case like a car trunk for the smaller actual case. BTW - I have lots of nice foam available if anyone wants to go the foam route.
How does the hardware attach to the case? Is the hardware kit a standard item - I didn't see it in the obvious places on their site.
I did not personally go this route, but I did research it on the website, by email, and by talking to the Caribou guy in person at the MOA rally. You need the following DIY kit, then use the template to drill your case.
For some reason the website only shows the kit in two packs (presumably for saddlebags) and you want one kit for a top case. Anyway, contact the Caribou guy by email, he is very flexible and he'd probably sell you a single. If not, I'll split a pair with you. I want one anyway to attach a long rifle case.
After doing that you will have whatever size case you want and a lockable Givi attachment mounted to it. You'll still need a Givi adapter to convert the GT rack to a Givi rack.
By the way, if Caribou does have a size that suits you, I would definitely go with the Caribou bag. He adds a lock to the case itself that is key matched to the mount lock, so you have a single key to either open or release the case. If you DIY, you'll have a key to release the case but you'll need a padlock for the Pelican with its own key (or combination) to keep it securely closed.
One last thing, the locking mechanism to mount the bag to the bike isn't itself terribly secure. Someone with common hand tools could disassemble the mechanism and make off with the whole locked case. This is fine for stops and breaks, but I certainly wouldn't leave it on the bike, out of sight, for more than a lunch break or so.
One laster thing, the Caribou guy IS willing to sell a key-matched lock pretty cheap if I recall. Thus you MIGHT be able to drill your Pelican and end up with a key-matched open and release, but he suspects it would be difficult to do. For his bags, he uses specific jigs that he made and they won't fit other size cases. (I asked him to do a custom size lockable case for me and he explained why he couldn't.)
Hope this helps. Keep me in mind if you want to split a 2-pack!