This is the Thwacker Hammer.  It's inspired by one of Adam Savage's shop hammers and it perfect for knocking together printed pieces that need a little extra love.  This hammer is weighted so the handle is sturdy, but fairly light and the head is done with solid fill.  Overall it's fairly light, but it feels like a very solid hammer when in use.  I've included an optional cover you can print in TPU that gives it a rubbery cover to be even easier on your parts and even easier on the hammer head.  A fun project, and a useful thing to have in your 3D printing toolbox.