I would dispute 'practically nothing', but uby has a point. We had a folding changing table from Ikea that was basically two horizontal tubes with fabric slung between, stretcher fashion, mounted on X legs that scissored shut for travel. The fabric had fabric 'walls' at top and bottom to make it hard to simply slide off (and to stop any...liquids from running on to the floor).
Something of that design might be in order. Unbleached cotton ought to be fairly harmless, though I must say that we used a cheap foldable-for-the-nappy-bag changing mat on the top so that we didn't have to wash the fabric all the time.
I've got weight distributions for children up to five somewhere. I'll post likely weights for you later, if you like.