The pole with a bronze serpent mounted on it prefigured the cross by which we were saved from death.
The tree of life in the garden of Eden prefigured the cross (Gn 2:9), which in the preaching of the early Church was sometimes called a tree (e.g. Acts 13:29).
Moses' staff prefigured the cross in that it was used to redeem the chosen people as the Egyptian army was drowned in the Red Sea (Ex 14:16).
The firewood that Isaac carried on his shoulders up Mount Moriah prefigured the cross, for the wood was to be used to offer Isaac himself to God as a holocaust (Gn 22:6, 2).
Noah's ark and the ark of the covenant were two different kinds of wooden containers, both used to save God's people (see Gn 6:14ff; see 1 Sm 4:3).
"Blest is the wood through which justice comes about" (Wis 14:7). All six of these wooden objects prefigured the cross. However, no one ever suspected that the cross, a wooden implement for executing the worst criminals, would be the wood used for our salvation. Never cease to be shocked by the cross, horrified at the evil of sin, and awed by the love of God, crucified on a cross.