The contract doesn't/shouldn't be for a meaningful amount of work. It should be for the smallest amount of work that will allow you to evaluate the candidate. It doesn't even have to be for 'real' work - it could be a problem that has already been solved, or something that you think could be better but isn't a priority.

The point is, there's no reason for it to be a genuine contract position, and every reason for it to be concise enough for them to fit in during their off hours. After all, you are going to have to look at it too :)

And they do have the time for it: after all, they are making the time for interviewing.