I have built myself a couple of these. An old nylon chopping board, hacksaw or similar, small bolt at fulcrum, longer bolt and butterfly nut and spring. Butterfly nut makes it adjustable. Took a couple of goes to get it right, but virtually free - nothing spent other than time.
I have another made from the hook of and old wire coat hanger. Saw that one online. Probably my favourite as it rolls off easiest. A wrap or two of silicone tape. I have it on the throttle handgrip.