I have a GX680, but it's not clear what dimensions you want. The mounting flange for one of my lenses is 104mm x 102mm (or very slightly over 4" square) at the point where it mounts to the camera. However, it doesn't have a classic LF lens board design. Look at some up close pictures from various ebay auctions to see what I mean.
There is a LF lens adapter available for the GX680. I don't have one, but the googles will show you pictures of what they look like.
Are you trying to mount a GX680 lens to a LF camera?