As you have it drawn there:
That ladder looking thing you are using to support the plywood drop-in ramp is going to buckle at the joint where the two parts meet. I think it will probably happen the first time you put your weight on it. You need to tie the lower section of the ramp into the ground underneath the joint. I would keep what you have but add some 2x4 framework underneath the lower ramp section. When you're done it should look like a big doorstop, if that makes sense.
A non-exploded view would be helpful too if you have one.