Lift would be sweet. Not exactly in the budget.
I like your 'plain box' idea. I do not think shifting the weight to the outer edges would create a capacity problem. Simpler than what the Porsche guy did.
As long as it fit pretty tight on the sides, it would be very stable.
The only downside here is that it would not be as universal, in terms of fitting on a variety of ratchet jacks. For example, I have several brands and sizes, all with different saddles. Maybe a simple box with open ends, and setscrews on the sides. Like the Porsche guy, but somewhat longer to avoid the fussy contouring.
The rubber top is so that it can be used in other places on the MB, and on other cars.
1986 300E 5-Speed 240k mi.