Doubling this amount wouldnt be easy , as OP asked for on how to double this amount, it would really need investment though in able to double it but this doesnt mean that you would able to double it safely because as we all know that investment do have risk on lossing money. You could actually make some huge investments like creating a gambling site or an exchange site.
Maybe when the ZCash price drops to around $10, you can buy quite a lot of thsoe and double your money.
I think because of price increase it is hard to decide to invest right now in those altcoin and i think if he has patience it is better to invest in altcoin from top 20 altcoin and wait for a long time to increase each value.. and sell it in the right time..