GL4 IRH Focus Cap for 21.8mmx8mm flat convex lens. WORKING PROTOYPE that may require light sanding/resizing to fit, do not force anything. This is a 2 piece friction fit design. 

For assembly, I recommend sandwiching the lens into the adapter tube with cap (there is a very small gap between adapter and cap) before pressing the whole assembly onto the light head. This ensures the lens is flat against the light head, I messed with offset and found that with the lens I used no offset was best. Some tape or glue will keep the cap from dislodging and the lens coming loose. There are multiple caps with different sized openings to further reduce spill.