Yes, a caul is simply something to back up your clamping pressure - it can be as simple as a piece of scrap wood on your bench or something carefully shaped to fit a particular piece. Usually I make the shaped ones out of scrapes of MDF or press board - I use a router, chisels, rasps and sandpaper to shape them - sometimes I pad them with some thin cork (buy at Lowes or Home Depot). For really wonky shapes you can get that need moldable stuff from StewMac - would be good for things like bridges.
Sometimes making a good caul takes a lot of time but it is worth doing it. A very important one is the inside of the bridge area - make it while the top is off the guitar so you can get it just right.
Here a a couple that I made early in my building days - the two on the right are the inside and outside bridge cauls for a fan braced classical, on the left are the inside and outside bridge cauls for a pyramid bridge, with one for the fretboard extension (the notch clears the truss rod). The back are a couple of simple curved cauls for clamping braces to top and back and a couple of angled ones for things at, well, an angle.