If 5200 was the sealant..
BE careful removing your hatch. You might end up ripping the gelcoat up and doing damage if you aren't careful. I know opinions differ, but 5200 is not allowed near by boat. I learned the hard way.
When I removed the hatch on my Precision 21, I used a new razor knife (long retractable blade) and 'sliced' and as I pulled the frame up. It took a whlie and I still ripped the gelcoat from the fibreglass in a few places. Repairing the damage isn't a difficult repair but it does take time to get everything smooth and even. Besure all of you old sealant/adhesive is removed before installing the new hatch. I had my hatch a head of time and it still took 2 days, due to the gelcoat repairs, to get the hatch back in properly.
I repaired my gelcoat damage by using resin and thickening powder. Marinetex or any 'marine' 2 part putty would work. Avoid 'Bondo'.
Why can't you measure from the inside. I was on a H22 20 years ago but I don't remember the hatch. Is there a 'trim' piece that could be removed to so you can ge a good view of the opening?
I have a friend with a H22. I'll mention this thread to him.
Good Luck,
Mike