How to join two pieces of sheet metal?

Pop Rivets

The easiest way would be with a pop riveter.

• Clamp the two pieces together. Drill a hole for the rivet. Install one rivet.
• Repeat this for a rivet at the other end.
• Drill all the inbetween holes.
• Run a strip of thin cardboard between the sheets from the uphill side to get out any shavings you missed.
• Rivet the remaining holes.

I suggest practicing on scrap. A pop rivet has one side that doesn't look bad, but isn't pretty from the other side.

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Epoxy

A second easy way is with epoxy. Read the labels and get one matched to your metal.

Epoxy needs clean metal to bond to. It also helps if it has some tooth.

• Do a dry run so that you know your clamping system works. You can test it by clamping with a few strips of paper between the sheets. If you can pull out any of the sheets, you need to redesign.
• Mark the limits of where you want the pieces to align with a scratch using a utility blade. You do not want to go through the paint or galvanizing if present.
• use a 100 grit sanding sponge to rough up the surface and take off the shine.
• wash once with tri sodium phosphate, 1 tablespoon in a quart of HOT water.
• Rinse with clean water.
• Allow to dry.
• Wipe with an acetone soaked cloth. (Caution moderately toxic and very explosive fumes.)
• Let dry. (Won't be long)
• Mix up your glue. DO THE FULL MIXING TIME
• Apply your glue in a thin layer to both surfaces. A wood coffee stir stick works well as an applicator.
• Clamp the pieces together.
• Remove any glue that squeezes out.

You can try using construction adhesive meant for non porous surfaces. This is hard to squeeze flat for a short joint, but it may be more tolerant of temperature changes or outdoor use.