Knowing what I know about construction nothing you're going to "rig up" is really going to be adequate for proper ventilation.
That being said with all of the CS vents open a good fan in the doorway should be as good as anything simple.
As for a "trap" or backstop, I use something like below, which would work in a situation like yours as well as outside in general, and can be made bigger pretty easily.
View attachment 802903
That's basically a 4x4' piece of OSB on the front and back of three 2x8's, lagged to two 6x6's and filled with sand.
If you're mainly shooting .22 it'll last for quite a while. When the front sheet of plywood starts to look ragged just screw a fresh sheet over top of it.