Okay,
@ChicoryBlue here goes. I hope I explain what I am thinking clearly enough to understand.
Starting with your 2X2s, keep them as is - but the exact with of the plywood apart, so the edge of the plywood is even with the edge (at least the front edge, anyways - if the fence spacing doesn't allow both front edge and back edge to be even) of the plywood.
I made a model from cardboard to illustrate most of what I'm thinking.
View attachment 3369588
See how 2x2s are spaced to be even with edge of plywood platform below:
View attachment 3369591
On the under side of plywood, attach strapping/ 1x2s so that when you place the plywood on the 2x2s, the strapping fit snuggly inside the 2x2s:
View attachment 3369599
This should help hold the plywood in place so it doesn't slide, so no need to screw it to the 2x2s.
Then, for the ladder
(I'm not sure if this will work with your ladder, as I don't know just how wide the 'hooked part' is that would slide over the below set-up)
Can you screw two short pieces of 2x2 to the outside edge of your cross bar 2x2, just a bit farther apart than your ladder is wide, then screw a piece of 1x2 to that that you can hook the ladder over? Examples of what I mean below...
View attachment 3369606View attachment 3369603
Above is a 2x3 screwed to 2x3 'spacers'
This gives me something to 'hook' the ramp to, below images:
View attachment 3369605
I actually hook the ramp to the door handles I put in an angled edge (see middle picture), but I know someone who did something similar, but hooks it to the board that is spaced out from the frame.