How can I purify myself from major ritual impurity (janabah) while having a cast on my foot that prevents me from washing my entire body, for fear of damaging it or worsening the injury?

A person in a state of ritual impurity (junub) who has a wound covered by a splint should wipe over the splint and wash the rest of their body if they are able. If they fear damage to the splint or slipping by placing a bag over it, they can take precautions to prevent water from reaching the splint, or wipe around it with a wet hand, or place a bag over the splint and wipe over the bag.