I think the most acurate method of customising your centre spot filter would be to place a clean piece of paper into your easel, illuminate it with your enlarger, and photograph the paper with a camera which produces negs in the size of the filter holder. Use the resulting neg as your filter. This should give a proportional fall off on the filter that matches the fall off on the the enlarger. You will probably have to experiment with neg density to get it right.

Of course, you will have to produce a filter for each format film you use in your enlarger.

Graeme