O.K., Ladies and Gentlemen. Here are the sketches you can use (try) to make the holder. All dimensions are metric and taken directly from the pictures or extrapolated from there.
Please note the following:
First, the alloy spare is mounted the SAME WAY as the steel - inside up! NOT outside up as mentioned earlier.
The longer center hub of the alloy wheel requires the holder to be "shorter" for the wheel to clear the truck lid.
As Pat (Shvegel) said, the most difficult item is the round top, which is hollow in the original part and probably has a thick round plate welded to the top from the inside, which carries the threads for the clamp bolt. This can be made from a solid round piece with a hole drilled and tapped (heavy) or a tube type part with a flat disc on top and a M12x1.5 nut welded to it.
The length of the cylinder should not exceed 60mm in order to achieve proper clamping. The bottom has to have a lip which has at least 75mm diameter in order to make the wheel hub sit on securely. For ease of manufacturing, this could be a separate ring welded to the cylinder from the bottom.
The rectangular box profile post is 60mm x 50mm for the original part and due to availability of stock material can be made of 2" x 2".
Wall thickness I have assumed is 3mm or 1/8"
The post needs to be slash cut on both ends to achieve a lean angle of 55° from vertical leaning to the left when viewed standing behind the car. This is necessary for the wheel to clear the right rear quarter panel, similar to the lean of the steel wheel holder.
There is another slash cut on the BOTTOM of the post to achieve a 5° lean angle of the top towards the rear of the car in order to make the wheel slant confirming with the trunk lid.
The base plate dimensions and mounting hole positions are suggestions only and need to be patterned after the original plate of the steel wheel holder in your car in order to fit. They may vary slightly from vehicle to vehicle.
The only unknown is the hook for the wheel chock. From the pictures, I deducted it is welded to the side of the post which points towards the fron the car and it is turned by 90° compared to the steel wheel design. The wheel chock should bolt to it accordingly. I will make a few trial on my car and confirm there is ample space for the chock.
Look at the drawings and if you have any questions, ask me. Also, please point out obvious errors and missing points.
