There's always the option of a small frame inside the seat unit that could bolt directly to the standard subframe ?
I've not got a standard frame on mine so can't picture how to go about it tho sorry. I'm guessing it's do-able all the same.
I put an LC seat and tailpiece on my old Harris but made it so the original battery box, fittings and seat unit could go back on.
Maybe it's time to reach for the hacksaw
Then again, maybe it's time to have my tools confiscated !
Here's a pic of a bare frame I'd looked at for ideas before I bought mine.