I'd call Burley
We have a 2 year old d'Lite (I think it's similar to the current models) and have done various things over the past 2 years as our son has grown. When he was too small to reach the "helmet pocket", we put a small pillow behind him. This kept him upright and enough distance away from the back of the seat so that his helmet didn't get in the way. Now that he's a bit taller, we don't need the pillow and we adjusted the tension straps to give him back support. He sits in there very comfortably. If the solo has a similar seat to the d'Lite, I'd suggest that you just need to adjust the straps at the top and back of the seat (behind the seat in the storage compartment). If it doesn't seem obvious, I'd just give Burley a call - their customer service is very friendly and helpful (I called them to get a replacement flag after I got mine caught in the garage door.)