Below, you'll find the currently most popular Iberist flag, followed by two designs by Sammy:
Proposal by Sinibald di Mas i Sans (1854), with the colors of Portugal coat of arms (blue and white, also used in then flag) and the colors of Spanish flag (red and yellow).
Proposal #1 by Samuel McKittrick, with the union of both Spanish and Portuguese flags, and coat of arms of Spain with Portugal in highlight position.
Proposal #2 by Samuel McKittrick, with the union of both Spanish and Portuguese flags, and two stars for the two countries.
I like the designs, but I decided it would be fun to go on a different approach. Here's what I came:
This design is inspired by Union Jack and other similar designs. The blue cross on white was the first coat of arms of Portugal, and survives on current coat of arms of that country. The knotted (said raguly in heraldic language) red saltire on white is the famous "cross of Burgundy", used as merchant ensign (specially in former Spanish America), among other uses, and still present on the personal coat of arms of Spanish monarchs. I also made versions with the cross of Order of Christ in red and blue.
As a bonus, a what-if. Portugal and Spain had a joint candidate to host the 2018 football World Cup. If they won, I think this design, inspired by candidature logo, could be used to promote the tournament:
Your comments are welcome.
Don't forget to visit Sammy's post here.