Sorry to hear about your knock back from OASC. The big thing now is to show them that you have listened to their advice and have undertaken activities to improve your weak areas.
What can help with practical problem solving skills and can also give you something more to talk about in the interview is a week long working holiday with the National Trust where you build wooden fences/repair walls. It is pretty straightforward to get yourself booked onto one (via their website) and they are more fun than you think. Granted, the adventurous training expeds would be ideal but if you cant get yourself onto an exped, a working holiday will help.
All the best