The first challenge was to create a steel-rimmed roadwheel that matched the scale of the rubber-tired wheels on the plastic Panzer IV Ausf H set (GBX79). I ended up finding some styrene tubing that was the right diameter, though it was too thick for the steel rim of the roadwheel. I thinned it down quite a bit, but left a little thickness there so you could actually see the detail once it's done (as the "real thing" is fairly tiny). I then added the center detail and hub and cast up several (shown below).
I ended up casting the wheels as a fairly long tube and then sanding them back to the required thickness rather than trying to cast a very thin wheel on its own. This let me custom fit the roadwheels to the tracks. Using my Dremel tool, I carefully removed the first four roadwheels of both sets of tracks, and then added in the steel wheels as replacements.
Above you can see the process half way through - the top track run is ready for casting, while the bottom track run has had the first four roadwheels removed and ready for replacement.
At this point I have finished both sets of tracks and the RTV is currently halfway through its 24-hour curing cycle - so by tomorrow night I may be able to start casting new tracks. I decided not to cast the fenders with the tracks, and instead I'm going to borrow the plastic fenders from the Battlefront Panzer IV Ausf J (GBX68) and use the metal ones from the Panzer IV/70 (A) (with the correct idler) for the Panzer IV's (as the Ausf J only began to use three return rollers starting in December 1944). I'll continue to post updates going forward as the project progresses!