I have a low wheeled table that used to be my computer desk. After a move, I replaced the old table saw I left behind with a new contractor's saw. The desk got co-opted for the saw. Everything was fine except I didn't use locking casters originally for the computer desk.
I needed to make the saw stand more stable for a project and didn't have time to make a towing base or one of those foot operated retractable caster gizmos. Those work well and I plan to make one of those again one of these days. I didn't want to simply remove the swivel casters since the project is taking a while and the table needs to move out of the way when it's not in use.
Step 1: Drill Holes in Thin Square Stock
To hold me over until the fancy caster setup gets made or I buy some locking casters, I drilled holes through some thin plywood squares I had lying around with a Forstner bit. A spade bit would work fine too.
After drilling, you wind up with four caster cups.
Step 2: Lift Table and Rest Casters in the Cups
Place the cups on the floor at each corner of your table, lift the edge of the table up slightly, and manipulate the casters into the holes of your thin wooden squares.
When you make your squares, thinner stock will make it easier to lift the casters into the cups and will likely fit against "typical" casters better, but just use what you have handy. Heavy cardboard may even do the trick. If you have larger casters, then you may have to resort to a hole saw to make a bigger hole. Ideally each caster will rest in a cup yet be wedged against the edge of the hole you cut.
There's likely a formula out there to determine how large the cup hole should be relative to the caster size and the thickness of the stock in order to obtain the ideal ratio between the caster size, hole size and stock thickness. I just eyeballed everything and it worked fine. In my case, the casters are 2 inches in diameter, the plywood is 3/16" thick and the hole is 1 inch. Don't forget to convert the 3/16th common to actual. :-)
Step 3: Stow and Go
That's it. A nail or screw in your table will hold the cups while you wheel your portable table around your workspace.