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Hello chaps, I've spent half my life working beside noisy engines, so I've had to quieten them down. If you have a noisy outboard, your entire cockpit is amplifying that racket, and your main cabin might be acting like a resonating box too. Just two sheets of plywood will make an astonishing difference, one laid across the top and another to make a wall in front of the outboard.
Of course if you want to spend two or more hours making a box, with holes (or flexible tubes) for inlet and exhaust, and very thick sound insulation, the difference will be truly awe-inspiring. I often have to spend all day working beside a "silent" inverter generator, and under full load it's not the least bit silent. Some super-thick insulation and plywood made a hell of a difference. Of course this was an air-cooled engine, so I had to put in fans and a temperature probe to prevent overheating.
Even the most ill-fitting bits of plywood around your outboard will make a huge difference, so long as it can breathe. If your engine's air cooled you'll want to be v careful. If you have a stern-hung outboard, some thick insulation around the cover will make a huge difference, but the main thing is to reflect the sound away. If you can put a canvas cover on your pushpit/rear railings, or better still, some people have a seat on their rear railing, and if you add a wooden 'wall' beneath the seat, it will really reflect the sound away. Hope this helps chaps, you can calculate the decrease in decibels with the following helpful equations:

Ok Professor Chunderguts,

Lets see your workings for a 9 Hp water cooled outboard in a well.

Haven't got my slide rule, but I would make a removable box with heavy insulation that you can just drop over the top (it will get a bit wet from that low cockpit floor), and I'd make sure there was an air inlet allowing sufficient air to the engine intake. I don't know whether your exhaust goes out under the boat, or whether it must escape into the cockpit, but I would definitely arrange some sort of exhaust hose if it vents into the cockpit. I thought a '69 Mk1 would have an inboard boneshaker? And just out of curiosity, my Mk1 has an inboard (seized) BMW diesel - no evidence it's ever had an outboard well - does this mean I've got a Baker-built?
Thanks, -Will

Will,
Yes if you have some sort of proof it was built in 1969 and it has NO outboard well, then the chances are very high [100% ] that she is one of the 59 that Baker Built.

I think there are only 6 Formit Mark 1's, all built with an outboard well, in the second half of 1972.
I know where the first Formit Mk 1 is and one other.

While still obnoxiously loud, this cover just over the top of the outboard makes a huge difference.
https://m.youtube.com/watch?v=qlkVgw3rW34

Is that a poop deck on your Top Hat? That's a brilliant idea, I'll do the same. I'm gonna bung a seat on my rear railing - perhaps Phillip has one on his? I'm a huge fan of anything that can make the boat feel larger - BBQs and eskies hung over the transom, and if I put an outboard on, I'll hang the fuel tank over the stern to prevent fumes building up in the bilge. If you put a single piece of plywood in front of that outboard, even if it's a terrible fit - it will make that outboard unbelievably quiet. You can put it on a hinge, or put an opening panel in it if you wish, or just drop it into a slot. Cheers, -Will