I did this a few weeks ago but only just got around to posting it. I bought the carrycot to go in the new pram and well it was pretty plain. And the design is a bit stupid because it won't go in the washing machine (unless I'm missing something which is possible). So the lining is a black cotton which won't stand up to baby sick that well I don't think!
So I had a hunt and came up with some fleece and calico
Then I simply measured and cut and sewed the two sides together and ta dah a little snuggly fleece mattress for the carrycot.
Much more appealing for a newborn plus it will go in the washing machine