Willow or poplar(Cottonwood,Aspen) are good woods for backstops. Much better than the pines. The pines have too much pitch in them, and they get very brittle when they dry out. Pine's only advantage is that its much lighter to lug around than the other woods. If you are moving slabs, or the tripod, this is at least a two man job to move it to the back of a pickup truck, or take it from the truck to the tripod or stand. We build a sturdy wood shelf to support the slab, and don't use chains. If the shelf and tripod is angled properly the weight of the slab prevents it from tilting forward when you lever your axe out. You learn to push upward on the handle, and not downward to remove the axe.
I would not put screws, or u-nails, into the side of the block. We put them in the back of the slab, so that we did not ruin anyone's axe by having anything metal that could be struck by the axe if it was overthrown.
Games: The sky is limited only by your imagination. You can put a picture up of Osama, or Hillary, or Obama, or McCain, or Bush, as you wish, and throw at that. Pick a part of the image to hit to score extra points. Same thing with cards. YOu can decide that if someone hits the card and cuts a given " spot ", they get more points. ( aim small, miss small) We cut rubber bands, with 5 pts for cutting one, and ten points for cutting both. The bands are run across each other to form an "X" from 4 nails. You can use the standard bullseye, but use a die( or pair of dice) to determine if the axe is to be thrown overhand, or underhand, or side armed. That tests those skills, and gives members a chance and reason to practice them. You can do a dual tree, where members are paired off with other members. The loser of the throw goes over to the loser category. The winners then compete against each other, and the " losers " compete against each other. The winners in the loser column move over to the winner column. Only if someone loses twice are they disqualified to continue. That makes for a longer competition, but it also is COMPETITIOn on a man on man setting, rather than each man against everyone else. Then you can do the same thing with a partner's throw. How you score a partner's match makes it more interesting. If the first partner misses, but the partner hits, the two go on to the next round. In effect, a partner can save his partner and himself by picking up a target that the first man missed. ( failed to stick his axe in a scoring area). Or you can run a match like an Annie Oakley match at Trap. Each man gets to throw at a target. If he misses the next man throws. If he misses, the third man throws. If the second or third man hit, the first man gets a point against him. If the second man also misses but the third man hits, both the first and second man get points against them. 2 misses where points are awarded against you and you are out of the running.
That's enough ideas from me at this time of night.