Flexible, Light-weight “Training Wheels” for Adult Bicycles

I would love to have relatively inexpensive traing wheels – for want of a better term – that I could retrofit onto my current two wheel bike (a Specialized roll with low entry) and have as a back0-up when I feel unsteady on my bike.  In other words, I would like to be able to apply and retract them using controls from the handlebars.  I would like them to be safe, relatively cheap and very lightweight so I can continue to load my bike on my bike rack by myself.

I know three wheel bikes like trikes and recubent bikes exist.  However, I believe trikes are heavy and unwieldy to lift onto a bike rack by myself and recumbent bikes are heavy and expensive.