Here is a clamp-on that goes over the existing boss on opposite side,
http://roadbikeshifters.com/29-99-si...ur-campagnolo/
THere were somewhat commonly used back in 50's/60's to upgrade older bikes that came with antiqated front deraillers that shifted via linkage rod on the seattube instead of via cable. During this era, plenty of nice bikes were made with just a right side shifter boss and when cable pull FD became cutting edge old bikes needed a similar shifter clamp to add just a L-side boss.