As far as I know there are plenty of bonuses out there that don't require you to deposit your own money, in fact here on bitcointalk you can find free bitcoins in the giveaway section, I've seen plenty of prizes/contests/etc that give a small amount of free btc 10-100k satoshis for doing some really simple tasks, you can easily use that money to gamble, again risk free.
An example of that is Bitsler's contest, they have a wagering contest for their play money but you're only likely to win if you're a high roller because the amount of play money you get is based on your total wager.
Another is Stake's challenge, it's announced on their telegram you can win without gambling a single satoshi but there is one requirement which is to send messages on chat and to do that you need to gamble a certain amount.
Ok I will be adding Bitsler's contest and their free lottery as well in the OP, also Stake's challenge. Both are basically free so they should work, even if the first one is unlikely to happen, it's still better than wagering with your own money, sure high rollers are wagering a lot but they can also lose a lot to win a not so big prize considering they are wagering dozens of bitcoins.