Ok, go to the shops and buy yourself two things: 5cm (roughly 2 inch) wide cotton crepe bandage and some velcro cable ties (or roll of velcro).
Wrap the crepe bandage around your shaft, under the glans and use velcro to stop it from unraveling then apply your
extender. Also, look into using a velcro strap to secure your
extender. It's kind of hard to explain it but there are threads on this if you do a search (or if someone is nice enough to link to a thread about it)...
Your dick will still alter its appearance after a while and you'll still get odd bumps sometimes and maybe a thicker glans ridge, it all depends how long you wear it, how often etc. It's something you'll need to work out with your dick as everyone's different when it comes to pressure tolerance, recovery etc...