The only ledge that will be large enough to fit my Darwin's whole body on it when full grownwas test with roughly 7kg weight in a bucket and sat on the ledge with no probs. and that was before it was put in the enclosure so it will be even stronger now that the sides are fixed to the enclosure. Not sure if paper mâché would help, but in this case it doesn't seem necessary. I did have similar concerns at first, but 2 layers of polystyrene and real thick render seem to have it covered. ( for 7kg anyway)