You know as much as I do about the rear applications, I would suggest speaking with John Carey or Bob Santoro (if Bob is still around). BTW, Carey had tops in stock as late as 2002, I haven't tried him recently.
Oh Oh...breaking news!!!! northeastbritishparts.com lists tops (black only) for 583. and fit kit for 45. Pricey but less than UK.
They have a sp250 section on the website. If you order one, please PM me with details/result.