The filter on the inside rails is something I never thought of either. Rangefinders and tlr's would be easiest as other have pointed out. I have the Nikon AF-1 gel filter holder. It's an old piece of gear, it has a 52mm male thread, a foam lined interior (which I had to replace) and a odd outer female thread which I had to look up just now which is 60mm. You can use it for an ir filter two ways, find an IR gel that's not ridiculously expensive and tape it to the flip out side or use a step down or step up filter converter and a seperate IR screw on filter. This will let you compose and focus with it open, and shoot by closing and locking down. Remember to adjust your focus for IR.