We place T posts every 8 feet.
For a shelter, I’d do something similar to the pic. That way two goats can cuddle together and will have more room when it rains. You will probably want to have an actual floor though, the first will get gross quickly.
How long are you wanting to keep the goats in the 15x20 pen? Nigerians will be better than Nubians in this type of situation. Nubians get big (our doe has weighed of to 165 pounds). They have a LOT more poop, not trying to be gross but when keeping goats in a small pen you have to think about these things. Their feet will churn it into a mud pit once winter hits. Plus they will be costly to feed. Nubians eat. A lot. They will essentially be in a dry lot, so you will need to provide ALL feed and hay, they will not have adequate forage so that means more $ spent in hay and feed.