The pit needs to be big enough for the bed of hot coals, the pig and at least 6 inches of space above and on the sides of the pig.
I've not used rocks or bricks in my pits but they would help hold the heat in longer.
Holes in roofing are fine if you can cover them so the dirt doesn't fall into the pit. They might be small enough holes that it wouldn't matter.
Siding works if it's not painted or treated.
I've not used plywood, the pit gets pretty hot, not sure if I would trust it.
I have placed 2x4s across pits and covered with tin to reinforce it.... Works pretty well.
After digging the pig up the next day, it still needs a rest time just like any meat.
Another thing.... place the pig on it's back of possible. The skin will hold in the juices as it cooks. An underground pig's skin isn't crisp like a roasted one but it helps protect the meat and keeps every thing moist.
I'm leaving for a campout either today or tomorrow. Just wanted to let you know in case I disappear from the forum. I won't have computer access for a week or so.
Good luck with the pig, I hope all goes well for you!!
Hope you have great luck with it!