my DD1 has just finished being in a cast after 9wks (she broke her growth plates in her thumb and mey in the future need surgery as she is still in active growth phase)
we managed just by me washing her hair while she lent over the bath. it was not fantastically easy, but I figured it better than forking out for something that may take weeks to be delivered (even tho it claimed otherwise) and then she still not manage herself as she had no movement in her hand because of the way they had casted her IYGWIM.
bathing was us wrapping her arm in layers of clingfilm and a plastic bag and me helping her with the soap etc.....thankfully even tho she is 14 she is not too modest about me being in the bathroom with her. Her sisters also helped if I was busy with the boys....the few times she has been really happy about being in a large family.
I have to say, it was easier coping with this break than when she broke and dislocated her elbow 2yrs back........that was a cast up to her shoulder!
I used spray shampoo for my DS when he broke his leg. He was only 2.4yo so couldn't keep his leg out of the bath, so we gave him a full wash down every morning and then every couple of days we'd use a spray shampoo- i think it was called 'Bastille' or something, found in boots, cost around £3 for the aerosol tin and lasted the whole 4 weeks he was in cast and longer!!