Look at EZ-Lok. They make flanged and non-flanged versions. I picked up a few to get started at my local Hardware store. Found them in the little bin boxes, but there is a price for convenience. McMaster.com also carries them. The 1/4-20 come in two lengths but the short 1/2" long will go in from the top and leave some room for surfacing. Just make sure you put the all the way in to the bottom. There is a version that installs with a hex wrench and may be easier to install, but have not located them yet. The T nuts are much cheaper, but must be installed from the bottom and you may want to put a snall relief bore to let them sit flush with the bottom surface.