Temporary portable bathrooms during peak seasonal times

We are year round spice processor but have a seasonal dehydration processing where seasonal employees are hired for 5 months out of the year. We have looked to expand our current bathrooms to further accommodate the influx of seasonal employees, but it is too cost prohibitive at the moment. What is the standard for using portable restrooms for as a temporary supplement during seasonal operation. I know we would require hot water hand washing stations and something ventilated, idea is to put unit in front parking lot. Is there any more requirements I would be missing? 

I've had to use them temporarily.  Just control them as you would normal toilets.  I.e.:

Have a good flow, so handwashing is hard to miss.

Clean regularly.

Insist workwear is removed before use.

Provide plenty of paper towels or really good hand dryers.  You don't want an excuse not to wash hands.

Consider where the waste goes to drain, i.e. not into your product drains under the factory.