Here was my solution. A piece of 1x3 wood and 2 2" U bolts with plastic tubing to prevent scratching and slippage, and washers and wing nuts. Easy on, easy off.
I installed it on the upper bar so I could keep the seat set high for my wife. It took about 3 min to put on or take off. Also installed a bottle holder that provided clearance for the cables. I have a post somewhere in this forum from '09 with more details.
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Used it for 2 years and hundreds of miles, but now my grandson is tall enough to reach the pedals. Saving the set up for the new 1 y/o grandson.