How big a print are you making?
You said you were using a 1000 watt bulb so why would you need a center filter? You can get very good and even illumination from a 1000 watt MH bulb over an area that measures roughly twenty four inches square, with light fall off being very moderate up to 36 inches square. I print 8x10 POP with no issues what so ever and I never have it centered under the bulb. Might be easier to find a good reflector on the used hydroponics market, than to build a center filter and try to calibrate it.