With MDO, you'll have some extra time in prepping it, but it is a temporary sign. Sooo.......
You buy a pre-painted sheet, cut it 3" wider then you need (75") and use some decent wood putty on the top & bottom, as the sides will be buried in the posts. Use your thumb to put it in the voids and cracks. This'll take about 10 minutes. Give it a coat of top coat paint. About an hour later, give it another coat. That hour in between, rout out your two posts each 1.5" deep. Might take 2 passes. Depends on your router. So, In an hour, your posts are done and so is your panel. Now, letter just as you would anything else.
Assemble your sign on the ground (put some cardboard down first) Dig holes directly where the posts ends are and go about 38" deep or 12" below your frostline. Ours is around 24" around here. Once the holes are in, set the sign up and into the holes and like mentioned.... tamp, tamp, tamp until it's firmly in place. No need for cross braces, expensive ACM or anything else. Do not frame the top & bottom.