What an amazing idea - thanks for the link Naomi!
You are quite right about the price of wood, I was suprised at the cost of it when I went into Focus and Homebase but I bought a couple decent sized shelves from Ikea for a couple of pounds each :P
All my hamster cages are made out of 'really useful boxes' so i'm not sure these would be compatible as theres no vertical support (legs) ?
That said, there isn't any in a barred cage either, so I don't think i'd have any problem
Going to have a look through their other shelves now thanks Naomi