An old leather box type suitcase might look a bit twee on the luggage rack of a three wheeler and the leather is quite heavy
How about a Large Duluth Pack - something the size of the Monarch
It is made out of heavy duty canvas and has the nice leather and brass fastenings and copper rivets plus it has shoulder and carrying straps when taking the luggage inside or with you
I use Duluth packs for my canoeing and I put everything in a large dry bag inside
You can order them from Duluth Pack in a large range of colours to match up with your colour scheme although I have the ubiquitous dark forest green packs
You could get some nice leather straps or belts to strap the bag on or you could make some eyelets in the canvas on the underside and use bungies
I have used the Duluth packs on the back of my Morgan 4 wheeler and they look a treat but no photos