The safest way for you to store your specific Altcoins is to secure it by yourself so download the Altcoin's very own wallet that is giving you the right to hold your own private keys as no one will get access to your wallet in that way not unless you give it intentionally to others. If you want to hold it in a long term basis and to calm your mind you must never rely to store your cryptocurrencies to online exchanges even if it's a legitimate one. And when transacting online you must be vigilant in looking for the right URL because there are so many phishing website that is almost the same with the original one.