The best focusing screen suitable for your 500C is the Acute-Matte screens (Code 42215). There are two things to consider though:
1. this stuff costs some $300 (can almost buy another 500C)
2. you can't change the screen in DIY mode for 500C (possible for 500CM and newer models). Need to take out those screws and after installing the screen, supposedly calibration of focusing accuracy is required.
If these are non-issue, go get from CP.