I've just sold a trailer which I picked up for about eighty quid, it's generic made in china steel fabrication, I've tried to attach pictures, from above with cover on and from beneath leaning on wall.
This should be more than strong enough for a 26kg load, I carried up to sixty on mine.
However you will need to improve on the standard, the minimum would be to have a soft dogber on the sheet steel base and if you need to enclose the hound because there's any risk of jumping out then you'll have to get into fabricating hoops like on wagons in the wild west, then covering that.
Good luck! I can relate to this as our casual rides were always limited to four hours because we had to get back to guide dog.
Sadly you can't put guide dogs on trailers!