Just cook it for the whole time the roast is in there - put it in there before she pre-heats the oven and leave it in there till it cools down completely. Before you chuck it in, squish little lids of tinfoil over the inlets/outlets - that way it will stay sterile till you want to use it next.
I do mine in the oven all the time, usually just cook the crap out of it on 250+ for an hour or two, that way I'm sure everything is dead.