I'm using a 150XL with the centre filter and gels on the back of the lens. I cut a small square of black foam core and used an Olfa circle cutter ($5 - $10) to cut out a circle the same diameter as the rear of the lens. The gel is taped to the foam core and then pushed on to the back of the lens. Since you'll probably have a lot of foam core left over, you could cut several pieces and dedicate a filter to each piece.
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