Why not speak with the range management and ask what they require for shooting shot at close range?
If you use wood to absorb the shot, then you'll be stuck with lead impregnated wood that would then need to be disposed of.
The range might only require that you use a cardboard target backer placed high enough for the shot load to pass through it to reach further downrange. Usually it's only bullets that the range wants to avoid having ricochet off the ground by shooting at a target that's placed too low to the ground.
Most ranges don't mind shotgun patterning and don't expect that the shot be contained.
Plus not all of the shot will necessarily penetrate and stick into the wood target backer if the velocity of the pellets isn't high enough or if it's only light birdshot.
One of my ranges uses some very thick durable foam target backing on metal frames designed to hold the large foam panels. This about 4-6 inch thick type of closed cell foam would probably absorb the shot and contain it. Even plywood or particle board covered with a similar foam panel might work pretty good and it can't be very expensive or the club wouldn't use it. They probably get it at the big box building supply store. But still, where best to dispose of the captured lead shot when the entire range is probably already a contaminated property?
Any shooting club with a trap, skeet or sporting clays range isn't usually very concerned about spent shot loads because it's generally considered to be inert unless maybe there's tons of it.
Concern over spent lead bullets on club range property have generated nuisance lawsuits by neighboring residents which often fail once experts testify how harmless the lead usually is and the suits are dismissed only to cost the club enormous legal fees.
That's why it might be a good idea to find out what the range policy is about patterning lead shot and whether it requires spent shot to be contained or not.
If the club doesn't have a short distance pistol range, then spent lead pistol bullets may not be making it all of the way out to the sandy backstop/berm either.
This is all just food for thought since you know your range situation better than anyone else here does.