Hi Jurgen;
If you can't find the part you may want to make one. You'll need some styrene tubing and a bit of sheet. Bandai probably used a softer removable polyethylene material for the caps, in either case, here is an opportunity to try out scratchbuilding. Depending on your finishing objectives, you can always have the piece look fatigued or battle damaged.
I have a smaller scale Bandai Panzer IV, and believe me, it has a lot of battle damage and scratch built parts.
Given the time you've put into the kit, it makes sense to start making your own cap while you wait for good news from afar.
Regards,
Bockscar