Another idea just occurred to me: since these parcels aren't heavy, could you secure a cargo net to the back of the trailer (from the handle at the top, and the frame at the sides and bottom) to use a sling-type catch?

If you could load the sling through the space between the top of the trailer and the handle, that'd allow you to do a more-or-less permanent mount of the net with something like zip ties, or if you wanted more flexibility, maybe you could secure the net to the handle with hooks or carabiners that would allow you to detach part of it to get bigger packages in there.

The cargo net I'm thinking of is one of those types used in car trunks for securing parcels. Often they have a thick bungee cord or non-stretching cord around the perimeter which could give you something more substantial than the webbing itself to secure to the trailer.

Anyway, it's just an idea, but it may be worthwhile to play with.