My wife likes to move her large flowers pots around for variety and so they get equal amounts of sun. But the largest pots with dirt and plants are pretty heavy, likely around 60-70 lbs. Commercial plant caddies are usually kind of junky and virtually all have inadequate plastic furniture casters that don't roll right and quickly break.

Here is a design that virtually hides the stronger wheels, creates a platform look, and rolls much more easily than the commercial ones.

The most expensive part of this project will be the wheels. We picked up 4 screw-attached wheels at Harbor Freight for $10. They carry various sizes and types; we went with these 1 5/8 inch wheels:
http://www.harborfreight.com/1-5-8-eighth-inch-x-7-8-eighth-inch-light-…

If you choose larger wheels, they will no doubt roll a little easier over bumps but you will need to live with them being more visible or you will need to buy or cut wider boards than this design calls for to hide them.

A decision is also required as to what type of wood to use. This project will include two approaches, one simpler, sturdier, and does not require a table saw. The other approach is cheaper and sturdy enough.

For the simpler approach:
You will need 10 feet of 1 X 3 of the wood species of your choice. All boards are 2 1/2 inches wide with either approach.

My local Home Depot has 6 foot long X 6 inch wide X 5/8 dog-ear cedar fence slats for and incredible $1.39. One of these is enough for 1 plant caddy, with a little left over. There are 2 downsides to using this wood. First, it is rough wood; not finished on the sides. The other is its 6 inches wide and needs to be ripped to 2 1/2inch widths. But it's $1.39.

I am lucky and have both a table saw and power planer so neither of these issues caused me problems.