A cage is the modern standard, but it sucks in terms of aesthetics on old cars, so I still run my SCCA legal roll bar, properly braced and cross braced, and it looks nicely vintage in my old MG race car.
The Solstice is one of the harder cars to fit a useful bar into without rendering it a race-only car (there are harder - see Porsche 914, for instance)
You would need at minimum mounting pads welded to the frame for the main hoop, two rear braces at the correct angle, which would require chopping up the trunk, and a diagonal brace that would limit seat travel.
Listen to miller and check out what would pass before putting pencil to paper to design one.
1957 Jamaican MGA
1958 MGA Twincam
1962 MGA Coupe
1971 Jensen Interceptor
2007 BMW Z4M coupe
2009 Pontiac Solstice GXP Coupe
1965 Jensen CV8
1969 MGC roadster,
1969 Lamborghini Islero S
1988 Pontiac Fiero GT
Bill in BC