First off, this question depends on the part of the country where you live. If you are in a colder climate, that framing would not suffice to allow the proper thickness for an appropriate and code required R-Value insulation. If you live in a warmer climate, then that is obviously different.
You could look into 24" width insulation. The issue with this is that it is difficult to find 24" wide insulation that would fit flush into such a shallow cavity (2'x 8' = 7-1/4" actual cavity measurement). 24" R19 might work, if is compliant for you locality.
You would need to install somekind of vapor barrier facing the finished attic, if the insulation did not have one.
You could also attach nylon insulation straps to the underside of the rafters in order to hold the insulation batts up during and after the installation process.