68mm Downpipe connector. 
Made this to patch a gap from when I removed a waterbutt. It has a notch in the bottom which fits either side of the downpipe and directs water flow directly into the pipe. The top simply fits around the top bit of pipe. 

I've included cross-sections from the model below

Black parts are your current downpipe. White part is the printed part. It's currently 80mm tall, this can however be cut to length in the slicer or after printing.