This probably seems like an obvious suggestion - but have you phoned your local trailer dealership to ask this question?

I suspect you can buy the pieces for a rear tack, but the hard part will be putting them together because you'll have to make some adjustments to what you already have. For example, the removeable saddle rack in the rear will need a hole or some method of snapping into the floor. I would want to have a good look at how tack rooms are set up, and decide whether I'm prepared to make those adjustments to my trailer before purchasing bits and pieces.

Google Slide In Tack Rooms and see what you find...................