Great work building it yourself from scratch but I would copy this approach for less effort haha!!
He used this off the shelf lightweight trailer ....
http://www.carlex.com.au/index1.html?c35.html&1
Cut it down to match his trunk size and then with a couple of tie down secures a locking trunk but can also remove it to put a small dirt bike when taking littlies out for a ride.

Awesome job with your build and great to hear it's working perfectly.
Matt.