DIY Rudder Cap Retrofit?

Cheers all,
I'm currently doing a few DIY repairs on my Spec Ski - having now replaced the missing Bungee Cable to the Foot Pedals - I am now considering a retrofit of the missing ( rectangular ) rudder access hatch ( or cap ). It appears that nearly all modern replacement Caps are circular, whereas mine is square or rectangular in shape. I have a few basic ideas of what I might use to seal the open hatch without resorting to forming a new section with glass - some hardened plastic cut to shape would surely do. Any other suggestions from the brains trust before I venture alone?

Attached pic is what I am looking at ...

Best, Crab Stick

Why cover? I'd leave it open. Makes it easier to inspect for damage or other problems, and also avoids trapping water in there during storage.

I would certainly leave it open.  BUT, you have to fill in the 4 holes to disallow water from getting into the hull.  Permanent epoxy best, or can just put screws back in, preferably w/ some sort of caulk.  Water entering the rudder tube will simply run out of bottom, and water entering the steering cable tubes will come out the other end of the tubes or back where it came from, not the hull.

Thanks for the suggestions gents ... I'll just leave the lid off then - the previous owner stated the very reason the hatch was missing was for ease of inspection also; is this a common practice? I have replaced the missing screws to fill the holes with washers for now and will bed in with some silicon for future security. I had thought it would make the Ski look tidier to replace the hatch ... but looks aren't everything.

the previous owner stated the very reason the hatch was missing was for ease of inspection also; is this a common practice?

Yep, certainly is here.  I've taken the cover off every ski I've owned (except for the Carbonology boats, where the rudder shaft head sticks up above the deck.  It would be dangerous to leave that exposed in the surf, so on my Zest I left the cover on.)

Get a scratch resistance polycarbonate clear cover from McMaster.

www.mcmaster.com/polycarbonate