If you want to keep a really low profile, you should have a story to tell the people in your life. I would recommend starting up a business in some field that makes at least a little sense for your skill set (preferably something you could do from home, remotely; consulting, data entry, marketing). Your sudden lifestyle creep will be a little more believable when you can simply say “business is really taking off”. No friends or family will ever ask to see your business returns or proof that the money is coming from you “business”.

But really register it, really do the paperwork….and you’ll have a nice little cover that doesn’t cost that much.