Oldboot
The problem with retrofitting is that people don't use the righ glues, tools , maths materials or designs. They use stuff for things it was never designed for.
If you own a small tinny with no electrics go to the water and pull the plug and have a play.
If you look at the design changes the manufacturers have had to do it is substantial.
Changing original design can void your insurance
Can only suggest people get professional advice and have a good read on how water reacts inside a open topped container in water. If you can't understand the terms, maths and calculations get someone who can. Look past a millpond and go sit in the bath now with a tupperware container. Stick a bit of foam on it here an dthere and see how it reacts empty . full and with a bit of swell. Ya kids will love the extra parent time they are getting.
I'm off for a walk on the beach to grab a few extra thongs and broken boards to put under my tinny floor that isn't screwed down as I'll feel safer. If I have any extras I'll make up a thong and milk bottle life jacket.
I must ask if you think out=r floatation standards are good? USA has had better ones for yeras I think? Why aren't ours at least equal to them?