The picture shows another cross-section of the rim. Select the green edges all around the rim and begin extruding inwards as shown by the red arrow. Check your reference images to get the shape right.
Note: DO NOT cut your rim in half like I did, I just did it to make it clearer to see how I extruded the edges.
Next we have to make the holes in the rim. There are 10 holes which is why we created a tube with 20 sides at the beginning.Select the four polys shown in red.
Inset the selected polygons by about 2 as shown.
Extrude them inwards by -0.6 and then delete them. You have now completed the first hole in the rim. Do the same for the next four polys on the rim and keep going around the rim till you have made all 10 holes.
When you have created all the holes, your rim should look like the picture.