Another idea, welding a piece of box tube between the two upright at the height needed for the center ski and attaching your stolz to it for the center ski.......Then weld two more pieces of box tube at the height needed, one each side of the uprights extending out wards however many inches are needed and attaching your stolz for each outside ski.
If needed you can weld box tube too these new horizontal pieces and extend back towards a ski if needed.....
Or weld one piece of box across the two uprights and attach your stolz or extend towards a ski as needed and attach the stolz
Both my skis use stolz and extend out from my tool box