I have an idea running throuh my mind for some time, but can't get the time to get the rear wheel off and try if it's possible...
How about fitting 295x10 discs from Fiat Ulysse?
TRW 295x10 fiat ulysse Coupe 240x11 discs
TRW 240x11 fiat coupe 295x10, depth 56mm, c.bore 58 (trw sketch states 59?), holes 5x98
240x11, depth 40mm, c.bore 58 (trw sketch states 59?), holes 4x98
They are the biggest rear solid discs from Fiat-Alfa-Lancia parts bin (58mm center bore) that I could find. One would need to drill extra 4x98 holes, and make a rear caliper adaptor to offset the calipers 27mm radially and 16mm inwards.
The discs are 1mm thinner, but as min. thickness of Coupe discs is 9mm, it leaves 1mm of metal to wear...
I hope the rear brake compensator can cope with the increased size; I'm thinking about the mode because occasionally I park my car on a steep slope in front of the garage, and I'm not too happy with the stock handbrake performance.