Sorry for the delay. The platform is just a piece of 3/4 ply I had in the garage cut to width/length and the front corners cut off to let the front end fit between the air intakes. It just sits on the tire and is held in place by the straps--which are used to hold the tote, cooler, gas, whatever, as needed. When I'm not carrying anything I just tighten the straps to hold the platform. I have a 1x2 furring strip across the back so the tote can't fall off the back.
The roof and the back wind screen are cut from the same piece of ply. The roof is more or less permanent. I haven't decided, yet, what to do for a permanent platform or wind screen.
Since someone will ask . . . I don't turn very well when seated so I can't look out the back, anyway--I have side view mirrors on each side instead. The top section of the wind screen comes off for ventilation, as needed. Seemed like a good place to carry a flat pack of gas, too!