The answer is a silicone mould liner! My best advice is buy the mould and then get the box support for it. I bought mine and then got a friend of mine to make the boxes to support them. With the liners, all you need to do is pop them in the box, and pour your soap. Yay!!! No crinkles, easy demoulding, nice crisp corners. Honestly, these things are worth their weight in gold. Yes they cost at first but you use them over and over and over again, saving you time and money (because even if lining paper is cheap, everything adds up).
I bought my liner from http://www.themouldsshop.co.uk and it was a fantastic price and delivery wasn't too expensive. Anne is great and happy to help with advice... one of the great things about them is that she's made soap in all of these moulds, and has totally tested them so you can be sure that they work for all types of soap making.
And, even better news, they have added dividers for the moulds so that you can do Mantra Swirls!
And...they sell fantastic fragrance oils, not to mention glitters and mica and candle supplies. Honestly, this is a store worth checking out!
And no, I don't work for them but I do like their products and have used them and loved them so I'm just spreading the good news!
Check out http://www.themouldsshop.co.uk and say that Celine sent you via her blog ... Anne will be delighted to help.
(All photos courtesy of The Moulds Shop)