Old style mount

[Bait Bucket]

I was given a new, but older Big Jon electric downrigger from a friend. I want to mount it on a swivel base but there is a small problem, the swivel would be a 4x4, the base on the rigger is 5x2. Like I said, older style. I'm hoping someone knows what I mean by that and might have an idea as to an adapter plate for it.