I have done 3 DCL cruises solo and the best advice I can tell you is to book the lowest category available and enter 1. Typically the single supplement is 150%-160%. Also you might want to look into what OBC different TAs are giving out. They are typically given out per room and since you are going solo you won't have to share it with anyone. Lastly while on your cruise you also want to book onboard to get more onboard credit in addition to the TA's OBC.
Just get a small inside since your not in it much. Go off season. Look for agents with onboard credits. Vacationstogo will offer some discounts but it's usually the closer the date of sailing.