A custom cast is used to immobilise a broken bone when a pre-made splint doesn't fit. A custom cast includes several layers of cotton to protect the injured area, then the plaster or fibreglass cast. Keeping the cast clean and dry will help reduce the severity of your under-cast itch and keep the plastic from becoming soggy and shifting out of shape. If you do encounter it, there are ways to relieve itching under a cast.
Have benadryl handy in case of itching under your cast. Taking this over-the-counter medication may help relieve the itching.
Turn a hairdryer on the cool setting. Point the hairdryer down at the cast so that the cool air flows into the cast. If you keep the hairdryer trained on the cast for some time it may relieve the itching.
Shake talcum powder around the edge of the cast and use the hairdryer to blow it underneath the cast surface; the talcum will help reduce itching. Or use an itch-relief spray like Castblast that basically does the same thing.
Resist the temptation to stick something underneath the cast to try and scratch your itch. Doing so might injure your skin or damage the cast, ruining its custom fit. If the itching is truly unbearable, contact your physician about having the cast replaced or reshaped, or to inquire about other potential remedies.