Maybe cross-brace the inside? Matt would know better than me.
But, I've had a couple of ideas of how to 'skin' the bird, if you're unable to get fibreglass sheets or printing plates.
OPTION 1, use thin fibreglass matt, stretched over the frame, and apply the resin thinly. Once dry, a couple of coats of paint over a primer and it should be good enough, and fairly flexible to absorb any knocks.
OPTION 2. Cover in thick cardboard, then cover that in polythene sheeting, the type gardeners use. Tape all joints with duct tape, seal that with a thinned varnish, then paint.
Overall though, the printing plates, or properly applied fibreglass would be better.
Terry.