Go to your trailer manufacturers web site and look for accessories. You'll be looking for guide poles or other term for posts that rise up on either side of the aft end of the trailer. Look for "drive on"  style trailers specifically.  They are quite common in the center console saltwater boat series. 

Essentially they are poles that stick up above the water when the trailer is backed down the ramp so the boat will guide between them. They work good for locating the trailer behind the truck too.  Many of the saltwater types mount the tail light assemblies on these posts to stop that rust up non-working problem.  

 If the trailer is too narrow the posts will allow you to see above the trunk/tailgate etc but many small boaters put a horizontal piece of pvc pipe at tail light level so it extends out far enough to be seen in the mirrors. You can paint it, flag it or light it to be sceen. 

I recommend the verticle guide posts. The horizontal ones can get broken easily.    

for example..........

   http://estore.ezloader.com/subcatmfgprod.asp?0=200&1=232&2=-1

check out the ALS tube series...........

http://www.magictilt.com/features/brochures

You can fashion your own as well. Note that the stronger ones also prevent the boat from blowing off the bunks by wind or water current.

   There are magnetic trailer alingnment post with flags or ball tops that are used to help align the tow vehicle ball  and trailer hitch which might be able to ge attached to the trailer rails in the rear which might help. They would serve 2 purposes.

   You can consult for the trailer and also you can get customized trailer too, just check this out .... 

https://www.slideon.com/

When I'm towing my trailered Waterlust canoe I'm in the same doubts as to exactly where my trailer is too! When I look in the rear view mirrors, there's precious little of either to see.

So, after reading your post, I hit upon the idea of using a couple of driveway markers like these, sold at most any well-stocked hardware or big-box store, set into holes drilled in wood blocks fastened to my trailer's frame. At 48" they may be too long to avoid whipping a lot at hiway speeds but being fiberglass they're easily cut down in length if necessary. 

   If your car has a hatchback or trunk lid, could you try backing up with the hatchback lid open?  That's what I do with my Nissan Rogue, a pretty standard looking SUV.  I never back into or down a ramp, unless that hatch is open.  Gives mea  clear view of the whole trailer and boat.  Obviously you want to keep any gear secured and away from the lid, lest it fall out of your car while you're backing down the ramp.