You may be able to make a grain bag out of cheese cloth, which you can but at BB&B and other places.
The problem with a half batch is the LME won't store all that well. Might as well make the whole batch. Your beer may seem bad at first, but there's a good chance it will improve in weeks or months.
Are you doing a full boil or partial? If full, you need to make sure your stove top is strong enough and a wort chiller would help. But the instructions are probably written for a partial, and you can use your sink too cool.
Edit: If you chose the Midwest Newky Brown clone kit, it apparently does come with a grain bag but uses Dry Malt Extract, which stores better. Doing two half recipes will be more work on your end, but I can see doing it. You'll be able to brew your second batch sooner, and it's good experience to brew again.