I think extending the rental period would be really useful for both renters as rig-owners. Maybe the rig owner could set (or confirm) a price for example at the latest 1hour before the lease ends, which makes the rig 'extendable'. This way the future renter can pay in advance and carry on directly after the other renter, without 10 minutes downtime, the payment is already confirmed by then.
Or just a simple 'extendable? yes/no' option under 'my rigs'. Just like you can put your rig into maintenance atm.
'extendable? yes/no' is a good idea >> customer can see an "extend" button with the Mh/s a day price and just click "confirm" and do the payment, after the payment is received the time stacks on top of the remaining current time.