You can change the Z-scale of the doodad to make it very high.
To make it thin, it's either the X-or Y-scale (depending on the facing of the doodad).
Simple select the doodad (or doodads) in the world editor to change their scale.
You can base it off "Archery Target" or whatever fits your needs (it's possible to change the pathing texture, so it really doesn't matter which doodad you base it on).