Looking to build a mechanical cow setup

Recently obtained a started cow horse/ finished reiner. Would like to play with him and see exactly what he knows but unfortunately our cow source is out for the winter. I'd like to build some sort of mechanical cow/flag deal in our indoor for cheap since once it warms up we'll have our stock back. Any suggestions?

We have our flag system set up on an exercise bike and a bike rim. Of course someone has to be there to 'ride' the bike but it's controlled that way!