I'm pretty sure most have at least a square steel plate the same size as the base pictured. Or you can get short sections of tube to tie into the frame.
Most people on this forum would suggest that the OEM 'rollbar' is not stout enough to protect much at all. Metaltech (metaltech4x4.com) makes OEM reproductions that are thicker but look nearly identical and are not too expensive.
A roll bar or cage is only as good as the Metal that attaches it to the frame.. and on most of our rigs we are dealing with Thin & Rusty Sheet metal.. We always suggest tieing the Cage to the Frame with a piece of tube plated at the frame and cage