@SUP The " rim " idea is a good one .As to the pattern it is just one I came up with so nothing fancy but here it is as best as I can explain .
First picture is of it on a 1/2 inch grid if that is how you work . Second picture is my attempt at describing it . I used a 50 something year old tin class method of drawing mine , but a quick search on the U tube yielded a few ways . If you go the math route , diameter x 3.14 , the small arc is 4 5/8 but I made mine 4 7/16 for a tight fit. For me the lay-out and step method is easier and works just fine . The only thing I think this guy did that might not work is start with the larger arc and for my use I started with the short arc as it the critical dimension 1 1/2" , base can vary some ( larger or smaller )
Hope some of this helps .