Well. we've tried. From your description and the image, I assume that there is a chain running from each of the thighbands to the crotch, in which case I'd say that is the triangle (or the O as I called it before).
I hadn't realized that your chains were welded - that means that your father has no option to make it shorter or longer then, only to directly padlock the D-rings. But then I don't understand why your legs are pulled together when you sit down.
I understand that this has taken long enough, and you are tired of it, so I don't expect an answer; just couldn't resist writing my latest thoughts.
Maija T from one thigh band to the other thigh band and an additional chain from D-ring to the centre of the connection-chain.
This one for me
Interesting. I don't think I have seen that configuration offered as a standard item from the major belt manufacturers, but I'm sure they will do custom work too. I assume that configuration requires 3 locks, one to connect to each thighband, and one to the crotch. And if it were 2 separate chains, allowing the choice to not connect to the crotch, another one to connect the two chains. You mentioned somewhere most of your locks are integrated ones, I assume these, that would be better than having two padlocks on the inside of your thighs, but I'm not sure if those can also be used to connect chains to.
The picture is really small, but if it is just two pins that you stick in the holes of the lock, wouldn't it be relatively easy to create your own key with e.g. the tines of a fork?