Paco - I can fully understand why you would want to keep the back hair. It would mean no problems with keeping the wig bonded at the nape, which is one of the more problematic areas for full cap wearers. Technically what you are asking is very doable but there are cons to this approach. How bad are your sides and how far back does the good hair begin? Can you upload a pic of someone with a similar balding pattern as you? You might be able to go with a topper with large than usual temple flaps but it's hard to say without seeing an idea of your thin areas.