lumium is With a steam roller there a number of different types of rear roll construction in full sze,a lot of rollers for a start only used a single T ring as there was a lot less strain on a roller wheel compared to a traction engine, the spoke fixing to the rim varied,some cast in some rivetted and a few were bolted,this made rim replacement easier,spoke making can be a bit long and tedious,but thats what model engineering is all about.Now some makers in the later years of production after WW1 started using six spoke wheels,the hub and spokes were in one piece and bolted to the rim,Marshalls and Wallis & Stevens used plate wheels with circular holes cut in the plates,and this practice continued when both of theses makers started making ic engined rollers. so lots of choice, Should you wish to make a spoked rim then either get sawn off aluminium billets and machine either a single or double spoke flange,you might say a lot of waste material but it would only be a piece of ali say 4 inch dia and the wheel thickness,so easy to do and cheaper than castings.Or you could go for machining a plain rim from tube then then machine a recess in the rear of the rim to take another piece a smaller dia tube ,machine it into an angle rim and press or bond it or lightly press it in place with some small rivets to make sure it would not come apart, I would go for the single angle ring with spokes bolted in place, though I would go for machine the whole thing in one piece as I always try to make parts from solid, If you decide to make a double ring with rivetted spokes its not easy to form good rivet heads and if the rivets are hit too hard the ali can split. Castings in aluminium will crack even easier as I found out many years ago. Aluminium is a lot easier to machine particuarly as this would be a first attempt, My choice would be a roller based on a Marshall with flat steel wheels and make the model with machined from solid stee rims. Have a look at the photos of steam rollers on various web sites to see the various wheel types.