I used items from mocap in the UK (sent for a couple of free samples and got the RN 1-5/8CBK and RN 1-3/4CBK items) the 1-5/8" was a perfect fit and looks great but would have thought that there must be similar available in the US. See
http://www.mocap.co.uk/spec_sheets/pdf/RN-SQ-Series.pdf
and
http://www.mocap.co.uk/tubing-plugs.asp
They can be contacted on sales@mocap.co.uk but I think the mini pack contained about 50 and cost approx £30