ntsqd
technerd
I'd use a 90° pull De-Sta-Co clamp top and bottom.
Like:
http://www.mcmaster.com/#5135a23/=xic11h
Or:
http://www.mcmaster.com/#5135a41/=xic1sz
I'll also suggest that the tailgate fully 'nest' or have a 'V' section at/near the latch end that mates with a matching shape on the trailer. This will support that trailer side and keep it from vibrating going down the road.
Like:
http://www.mcmaster.com/#5135a23/=xic11h

Or:
http://www.mcmaster.com/#5135a41/=xic1sz

I'll also suggest that the tailgate fully 'nest' or have a 'V' section at/near the latch end that mates with a matching shape on the trailer. This will support that trailer side and keep it from vibrating going down the road.