Been awhile since I owned an NT, but I kind of remember a couple of bolts that are only accessible with the seat removed.
If your still looking for options....I ran a Seahorse case on my DR650 for awhile. Seahorse is essentially a different brand of Pelikan case, but significantly cheaper. You might find the size you like and be able to rig up something to attach it to the NT. I am actually a Givi fan. I have two of their topboxes now. I have used the V46 I bought on the NT on every bike I have owned since, including the DR650 after I took off the seahorse. I still use it on my Tenere when I don't have the panniers on. The panniers are top loading and the v46 is just wide enough to block them. So I had to get a smaller top box for touring with the panniers, but prefer the V46 when the top box is the only luggage on the bike.
The thing I like about Givi is that they make plates for almost every bike. So It is super easy to move the cases, between bikes, or buy it once and just buy a new rack for the different bikes. The Givi branded "racks" worked better than the SW Motech aluminum rack with givi adapter I had on my NT. All the other bikes got Givi branded racks and were a more solid mount.