I wouldnt bother with a USB, i would instead get 2 or 3 ledger nano S hardware wallets, which support Litecoin using the ledger chrome Bitcoin Wallet application. First you set it up and it generates a master seed. This seed can be used to restore your wallet if it is lost and you write it down on paper. You can import this seed into the other ledger nano wallets so have the same seed on multiples.
The transaction signing happens within the device and thus you can use it on a compromised computer without risk as well and you must click the button on the device to confirm any transaction and it displays the wallet address you are sending to.
You can have your BTC, LTC, ETH, DASH, DOGE and other wallets on here with the same master seed, you select the one you want to use through the buttons on it.
If you want to get one if i have been of help please use my affiliate link, no extra cost to you but i get 10% of the sale : - )
These are much more secure than just storing a wallet.dat on a memory stick as you still must load the private keys into RAM when you sign a transaction, with this you need not trust the PC you plug it into and the screen acts as a second factor authenticator. Plus you have the paper seed if all goes south.