No certificated GA plane is unstable in roll.

Uh, even an Archer is unstable in roll. I went out and tried it myself.

Are you using a different definition of "unstable"? Here's what I did:
I went up on a calm night, trimmed for straight and level flight, and
let go of the controls. The plane flew nicely for about a minute or
three, by which time the aircraft had gotten itself into about a half
standard rate turn, which kept tightening. It took a good few minutes
to get to that point, but it did so every time (and not always in the
same direction). So, on the larger scale, it is unstable in roll,
albeit not =very= unstable in roll. Small excursions lead to larger
excursions. It does require active attention to fly it.

Do you mean something else?

Jose
--
He who laughs, lasts.
for Email, make the obvious change in the address.