*Updated with more info
Basically, when you get that screen where you choose what item you want and it gives a x2, x10 etc bonus to get land on it.
Well what Ive been doing is
*when the item is selected RED
*hit the close{X} button that closes the window and you should get that item.
Perfect for any decor item you want : }
UPDATE
Ok so Jadd confirmed that closing the window has no effect on the reward. Just lets you get it faster instead of waiting for the random thing to finsh.
Heres a script you can type in chat to find out your Challenge Reward