I did some GW Saurus for a friend as a commission a long time ago, he wants swamp bases.
I traces the bottoms of the bases on plastic card and cut out as many as I needed. Then I cut the top flat area of the bases out and glued them onto the plastic card pieces. 30 models only took me like a half hour to do this. I based the inside of the bases with dirt bits, primed and painted the base and Saurus, varnished them after painting them, then squirted in the water effects and stuck some tiny fake lily pads and cat tails to the top of the water effects. Came out really nice.