To be honest I do not believe people will donate for cosmetics simply because it is a cosmetic. As of right now as you stated I see the only way is to donate for coins. Now this will also help me study for psych and this is why coins would actually be the best for you. Gambling is a variable ratio schedule meaning that people are sometimes rewarded after a random amount of attempts (attempting to roll). This is the most addicting type of I guess reinforcement, so people will continue to donate to gamble and roll 
. Thank you for helping me study.
*Note*- Guaranteed weapons actually probably wouldn't get you as much money as donating for coins would.
. Thank you for helping me study.*Note*- Guaranteed weapons actually probably wouldn't get you as much money as donating for coins would.