Looking at the picture of the back of your trailer…you could use the lower hinge bolts on each side plus the hole that the padlock goes through to secure the 2 bikes.
The driver side two lower hinge bolts would be utilized to design a bar/bracket that would stick out supporting the two forks via the axle holes (with tire removed). They do this often with bikes mounted to the roof of a car and use front axle quick releases on the rims. This would allow you to still see your license plate on the driver’s side in the stock position and not be blocked by a wheel. We know BC cops love to ticket for a blocked license plate. With the front wheels removed it also makes the bikes shorter so they do not stick out on either side of the cargo. Wheels could be stored inside so no one could ride away.
The passenger side two lower hinge bolts would be utilized to design a bar that would come out that would go through the rear wheel and the rear bike frame would sit/clamp on.
The top of the bikes would be stabilized be using a cross bar on the bikes top rail connecting each bike and terminating at the trailer using the padlock hole to attach it. It would be nice if it was two bars hinged like a clam shell around the top bike rails so the door lock would lock the bikes in place from theft.
OR
If you thought you had enough room under the trailer to mount a 2 inch receiver bar...go to Princes and by the plain 2 x2 tube, then have two pieces of angle welded to the top of it. One that you can bolt to the rear frame (yours is covered in a nice skin) and the other to the next cross member forward. In fact you don't even need the angle on the forward cross member as one long bolt through the 2 x2 and into the bottom of the frame will do…as all the weight is upward at that point.
Personally I would do whatever it took to keep them inside and free of mud and spray. That is what we did in the Boler trailer…out would come all the bikes and a full size propane Bbq when we stopped.
