Re: Older Toilet Seat Woes
One thing you could do is have someone (a woodworker) make you some custom supports that fill the gap between the modern seat and the toilet rim. With the toilet seat in place as you have it you could make a template of sorts by filling in the gap between the seat and the rim with some modeling clay or some other medium that will set and hold its shape. When the clay is set and firm you could take it to a custom woodworking shop and have them make you some inserts (for lack of a better word) and screw these inserts into the toilet seat (through the bottom) so that the seat is supported around its entire circumference. If and when you buy a new seat in the future you could unscrew the inserts from the old seat and put them on the new seat.
This could be expensive unless you make it yourself. The custom piece would be an unusual shape and require a bit if time to make. It might be cheaper to buy a new toilet. But thatís your call.
Maybe someone else has some better ideas but thatís what I would do.