We stock some hatch covers that fit the Seabird Designs sea kayaks:

Seabird Designs Oval hatch cover - fits the large oval hatch - sorry no longer stocked - no alternative available

Pacific Nylon 15cm hatch cover (small hard covert) fits the 15cm hatch  - comes with a rim as well
Kajak Sport Round 24 LRC - fits 10" hatch
Kajak Sport Round 15 LRC - fits 6" hatch (rubber cover)