You have engineering corps that are trained in construction.
But I’m thinking what’s required?
You dig a ditch.
You place the steel, with supports leveling it.
You pour concrete.
The border is 1900 miles long.
So you set up 40 teams, one team for each 50 miles.
Give each team 10 trenchers and 4 cranes.
Arrange for the steel to be delivered (Probably the bottleneck).
Arrange for the concrete to be delivered.
Set up a system, so the ditch is dug in time for the steel to be delivered and placed, and the concrete comes shortly thereafter.
Send in the Romans...
The Bees will build concrete plants along the way. It won’t have to be delivered.
Actually using steel - you just pound the steel sections into the ground to whatever depth is set - this sort of steel construction goes very fast and is relatively cheep. No concrete needed.