I think I could make something in aluminium but it would be time consuming as you say.
Starting from base zero, you have a Weaver or Piccatinny rail. You want to have an accessory pointing in the same direction as the barrel and set to one side, by what looks like 50 to 80mm, to be confirmed. The axis of this accessory is level with or just above or below the level of the rail, to be confirmed. How do you want to secure the accessory to the support? Could that be another short piece of Weaver rail and a regular scope mount clamp?