Slot-Based Inventory for Wagon

I’m creating a wagon inventory sheet for my players, who are using slot-based inventory tracking. They want to modify the wagon to have solid walls and a roof, like a vardo. How many slots should I reduce the carrying capacity by? I think the 3d6DTL crew did something similar in their Dolmenwood campaign. Any suggestions on the cost of the modification? We’re using OSE Advanced Fantasy. Thanks!