I think you can use any sort of rangefinder or antenna. I actually intend to modify a Din Djarin bucket with a movable rangefinder on the right earcap. There are ways to modify the 3D files, or to do physical modifications with resin casts and scratch builds. I don't think rangefinders are specifically denied to custom buckets; however, if you were doing something like a canon Boba Fett build, the rangefinder would be part of the requirements you must have before you could get approval.
Summary: better to rangefinder than to not rangefinder, unless you are building a canon kit that does not have a rangefinder. You are building a custom kit, so you should be fine here.
Also, Peej, based on your measurements pic and other sources, I would say your rangefinder stalk should be about 6-7 inches long.
As you hopefully can see in the picture below, the rangefinder stalk I have for the Boba Fett helmet, at 100% scale, is 6.76inches long. Even if your helmet is skewed a little in certain dimensions, or smaller or larger than a standard 100% scale helmet, it should be fine as long as you keep the rangefinder around 6.25-7.5 inches long. Go a little longer for a bigger helmet, a little shorter for a smaller helmet. The scaling is really subtle for this particular component, so whatever is standard should fit fine, as long as it isn't, like, a foot long or two inches long or something like that.
If you want to see the image more clearly, left click on it and click "open image in new tab".