Using a serrated knife, slice each ciabatta loaf in half horizontally. Spread the basil pesto generously across the cut side of both bottom halves, then spread the Calabrian chili mayo across the cut side of both top halves.
In a medium bowl, toss the arugula with the olive oil and a few grinds of black pepper. Set aside.
Divide the cheese and meat between the bottom halves of each ciabatta loaf and arrange in this order, overlapping the slices as needed: provolone, mortadella, Genoa salami, hot soppressata, prosciutto, and pepperoncini. Top each with the dressed arugula.
Close each sandwich with the top half of the ciabatta, mayo-side down, and press firmly to compress the layers. Wrap each sandwich tightly in parchment, then in foil, and refrigerate for at least 30 minutes (or up to 4 hours) — this rest lets the flavors marry and makes the sandwich hold together beautifully when you cut it.
Once rested, use a serrated knife to, cut each sandwich into 12 even small squares. Secure each square with a toothpick and store them in an airtight container in the refrigerator or cooler until ready to serve.