Don't know about Keifer trailers but... Trailer Body Components--Ramps & Ramp Hardware
There are ramps available as bolt-on that fit 6'9", 7'6" and 8' wide, standard length and then also as custom length from Sundowner. The ramps fit a variety of trailer manufacturers...
To me not terrible expensive to buy if it is a true bolt-on versus the $$ cost for materials, labor and engineering the proper angle of elevation needed for smooth and safe entrance and exit aka load and unload.
This is not something you can really just throw together when you realize how strong it must be made to withstand 1000 pounds per 4" square when a horse steps on and walks across a ramp...
Not made/constructed properly it will create chaos and possible serious injury besides be very difficult for you the trailer user to hate the ramp, and everything to do with it.
I don't mean to be a downer, but understand what is involved with designing, purchasing and physically making a ramp to do this job.
It is not a "simple" undertaking to make and make it last.
This will take special tools and skills of a metal working fabricator, a good one, so welds are a true blending of components and last, not crack and be defective.
Do your homework and figure out exactly what you want, what you need, what you can afford and what your trailer is capable of carrying weight wise hanging off the rear bumper location as it changes the geometry of the trailer design.
Good luck and please share project pictures.