If rent payment takes too long, rental should be cancelled and funds (with BTC tx fee deducted from them) returned to payer after his payment is confirmed - because it's renter's fault to make a mess with wrong tx ammount or fee - right?
I think a straight forward solution to this problem is to introduce an online wallet on betarigs -> Rigs can only be rented, if there are enough funds in the renters account.
It's unsafe for the whole service IMHO.