About this 3D model

My grandson is a great LEGO builder, but now I am giving him a classic Erector Set to expand his abilities. Along with it, I thought we could create a set of transition pieces that would allow mixing the two. I started with the library contained in http://www.thingiverse.com/thing:13531/#files but modified it. Read the instructions to see how to make the parts shown -- a connector from the LEGO block to a classic square girder, a connection from the square girder to the LEGO blocks, and a flat plate to screw on the top of an Erector plate.