You don't need your piece to go that far down the side of your head. If you get the piece to just cover the top of your head, you would be fine. Going down that far is going to be nearly impossible to blend with short hair on the sides unless you have someone professionally cut it in and blend and fade it.

I'd definitely not go with a lighter density. 50% you're going to look like you're going bald! You're a young dude! Just get a good cut in and blend and the density you have will be fine!