I'll second David's advice. I don't think a strip of fibreglass over the top would solve the problem.
A piece glued to the bottom of the seat, covering the split area by at least a few inches, would be your best option. Then, perhaps you could send back the top of the seat, scrape out along the crack to open it up a bit more, wet it with epoxy, and then fill it with a fairly runny mix of thickened epoxy. Wait an hour or two, and scrape off any excess so that you don't have to sand it off later. You might need to repeat the process, perhaps with some sanding filler instead of a glue filler, and then sand again, before coating the area with clear epoxy, and then repainting.
Peter