RE: Under seat rack on Giro 20
We only under seat rack we sell these days is the Terracycle Easy Reacher Rack and it will probably handle pretty whatever size pannier you have. The copy below is from the Terracycle web site.
The racks are designed to fit most panniers whose hooks are near the top. Many manufacturers are offering recumbent specific under seat panniers, making the selection even better. If you have any questions about how your panniers will fit, contact us. 1-800-371-5871.
Hope that helps!