Just a note that the land to build a wall is ALREADY ready and controlled by Federal government in the US-Mexico border in California, New Mexico and Arizona since President Roosevelt made the Roosevelt Reservation in 1907 that specifies that 60 feet on the border is reserved for Federal government in California, New Mexico and Arizona.
60 feet is plenty of space to build the wall.
The only issue with land is in Texas where government needs to use eminent domain on privately owned lands to take enough land to build a border wall. There is lots of government owned land and Texas state owned land also in Texas so this can be used to build a wall immediately.
Building a wall on the US-Mexico border in California, New Mexico, Arizona and Federal government and Texas state owned land in Texas can be completed in the next 12 months.
This would cause all the illegal immigrant crossings shifting to privately owned lands in Texas so most of them would take the eminent domain money quickly without lawsuits to get security instead of looking at groups of migrants crossing their land daily.
When using eminent domain it is clear that the wall should be built on the border and minimize the land taken (60 feet should be the max, I think 20-30 feet would be enough in most places) and the Bush era and Obama era mistakes of building a wall that traps some Americans and some American lands on the other side of parts of wall must be avoided.