Please understand Punctuality and other personality traits can not be judged in any Evaluation period. For a short period Human can evolve themselves as per demand. For mental, Behavioural & other soft skill traits if you can not judge a candidate in a day(single interview) then you can not judge him even in 20 days. Unless evalution is related to physical challenges along with mental toughness like any Army evaluation Exam, Sport Seletion, Space Exploration etc. this long period of evalution can not justify selection procedure.
However if you are still Ok with that i wonder what kind of Organization is insisting on expanding Recruitment cost where best quality of recruitment is possible in optimized cost & time. Instaed of planning such a long evaluation period you shoul mould your selection proces by inducing modern techniques like Psychometric Analysis, On Board Testing, Live scenarioo Handling, Group Discussion etc.
As per law outstation candidates must be provided with food & Accommodation. All the Employment laws whichever is implicit in your organization will by default be applicable on Candidates such as working Hours, Female employees fixed working hours with no late hours, proper breaks & sanitation facilities etc.
For drafting Evaluation Appointment payment terms should be clear & be mutually agreed by employer & Candidates.
Also Extra care should be given on Health & safety measures as there would be no insurance coverage for the evaluation period. If possible employer can also go for a group health insurance for that period if Testing environment is bit risky e.g. factory environment etc.