You might luck out and find some wave sport braces at your local shop in their boneyard. I would say all shops have old parts from warranty returns etc.. many are similar and you could modify to fit. If no luck there, spend some time on google looking for DIY thigh pads. There is a lot of articles out of the UK on it. At the very least you can shape some 1/4" plastic to bolt on and then glue closed cell foam to it and shape it with a rasp.