Hello,
standard SASS target size I think is 18" by 18" (for now anyway), for a practice target i like 10" circles. You can make targets out of 3/8" but unless you use A.R. grade plate, 1/2" will last a lot longer. Don't shoot jacketed ammo at them and they last a LOT longer.
There are as many shotgun knockdown designs as there are people who build them, google is your friend.
Good luck,
ML