I followed the advice robmac gave in this thread:
Originally Posted by bensonga
The best compromise screen I've found so far for the 1DS2 (or any 1D Mk1-2) has been the Ec-CIV screen for the 1D3 - pops right in and is nice compromise between brightness and prism 'pop'. Of course the sharper the lens is wide open (or at the aperture you focus at - I usually focus at my shooting aperture if light allows), the better the 'pop' in the VF. I've found the Ec-C IV screen to be excellent in my IDs2. The screen is comprehensively discussed in this FredMiranda.com thread.