I got a piece of 3/8" plywood, routed a hand hold, covered it with contact
cement, then slapped Astroturf on it.  It's not fancy, and it's not cheap.
(It cost about two bucks), but made a big difference to them.  Feel a little
funny when we put it out, but the two of them don't slip and fall, and know
when the green ramp comes out, they're either getting in or out.

Caveat:  Make it so that it can't slide.  I have a piece of cord on the top
the loops over a seat handle to keep it from sliding and causing a problem.

Look at simple solutions before you go engineering or buying something
complex.

Steve