So, an hour? Sounds pretty harsh at first but looking at the predicament he could possibly make it easier by taking some of the pressure off his thighs by using the fact that the hood is held in place and sort of "rest" his head in it. It might just help enough to make it bearable rather than comfy. That means something to make it "less" comfortable. How about tit clamps? Then the marbles. I like that idea. But I'd tell him he's got a starter of is an hour anyway, and for every marble that falls off would add 5 minutes of the fucking machine (face or ass, you choose). 10 marbles in total? Then tickle his balls.
That way the minimum he could realistically get away with is an hour tied, or if he's lucky, 50 minutes being fucked with an hour tied.