Replacing gully

I have a couple of gullies that take the waste water from hand basin and a kitchen sink. These are clay and square in section with a concrete surround that does a terrific job of catching the leaves in autumn. Now I know you can get covers for these things but I think they are unsightly and at almost a foot square a bit of overkill to take a 2" diameter pipe. So, does anyone know of an adapter that can convert these gullies into circular types with a screw-down lids?
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