Initially I awkwardly used one small lake next to a cave, then evaporated an underground resivoir to refill the lake. Then i used what small pools i found underground to convert some sand before it got to the surface to prolong the lakes lifespan as i funnelled the same through, and finally unlocked pipes to refill it from larger pools deep below.
Its worth noting that pile is after unlocking all research in the demo, so presumably actually made twice that amount in total.