It seems to me that the President and the Republicans (and the Democrats) have been wargaming these scenarios/strategies since well before the midterms.
As such, the Corps of Engineers should have made some preparations for making rapid contract awards.
They already had standing contracts with a pre-approved pool of contractors to (relatively) rapidly bid on each segment of wall. I believe that process was taking a few months of lead time for each segment (with several running concurrently).
If a priority list of segments was made available to the vendors a few months ago, they could have had time to prepare bids on each, and might be able to accelerate the contracting lead time.
To start work in 45 days, the vendors would have definitely have had to have been given a heads up to get ready before now.
If this has been cooking off for months, and they are getting ready to launch, it could be something glorious.
High ranking government people love to hand out contracts.