By law flaps have to be no higher than 8" from the ground. I don't recommend getting much lower than that either. Otherwise you will be backing over them in soft ground or backing up to a curb. I have owned trucks for 58 years and sill own a dozen trucks. And have paid for a lot of flaps over the years. I don't recall ever being written up for flaps more than 8", but every trooper has his own ideas of which laws should be enforced. I would recommend Buyers Products. They have a good product at a reasonable price.
The bolt on square stock holders sell for around $50 a pair. It is hardly worth trying to build something for that price. And not all flaps have the holes in the same place. On the square stock holders you can adjust the brackets to the holes. The TBOZZ E-Coating Black Mud Flap Hanger Bracket with Hardware Sold by Pair sold on Amazon will work just fine. I use Betts 3/4", They cost twice as much, but my trucks are on the road every day.
