After another fruitless search of the internet for a tutorial, I decided to work it out for myself (apparently, maths can be used for other things besides working out how much fibre I need to order and how much it'll cost!)
So, after lots of measuring, re-measuring, dividing and multiplying, I ended up with.... 2 foam rectangles as resists for a camera case and ipod case.
I won't pretend I ended up with 2 perfect cases after my first try, but they turned out okay and I found they are easy to adapt.
I re-did the camera case a couple of days later with 4 thin layers, and got the size and thickness I wanted.
If you'd like to try making a case with a flat resist, I made a picture set on flickr, here
And if anyone has any tips for making cases- I'd love to hear about them.
I also made a very simple camera case by sewing 3 pieces of felt together yesterday, and made a photo set for this on flickr, too
I must say thankyou to Linda from craft gossip who did a really nice article about me and my felted vessel tutorial :)
And if anyone has any helpful how to's or tutorials they'd like to share, please leave a link in the comments, I'd love to see them :)