This reminds me of my 1968 Corvette covertible.
Once you open the rear deck you'll have a place for storage.
If you put the top down, it will fold into the space but in a 'U' shape so there will still be space inside the 'U' where you can put things.
If memory serves, with the top up I was able to stand 6 or so grocery bags in the back of the vette as long as they weren't filled to the top.
I'm not sure how the top will work on the Solstice, but on the vette I had to pull a release lever for the back of it, lift the back up and then hit another lever to release the rear deck. Once the packing was done, then we'd close the rear deck and push the top down and catch it shut.
With the top down, the capacity was reduced to about 2+ bags with a wee bit of space under the folded top as it didn't lay flat on the floor of the rear deck...
From the pics, it looks like the back of the Solstice opens more than the vette did so there may be even more room...