My hens have no problems getting into my nests that are 2' and 4' off of the coop floor. Think Orpington, Delaware, and Sussex for size. The hens and full sized rooster have no problems getting on my 5' high roosts. They don't just jump, they also flap their wings and fly up and down. In my coop they have enough room to spread their wings and fly up and down without bumping into things. Some people use ramps so they don't have to fly.I have 4 nesting boxes in my coop already but 2 of my hens don’t use them, it’s ironically the biggest and the smallest hen that don’t use them. I have a theory that the smallest one can’t jump that high bc she can’t even jump up on the roost and the biggest one is just lazy I think.
Where are they laying, on the coop floor? I find a lot of these things are not that they cannot do things, they just don't want to. They may have decided that where they are now laying is where they want to lay. Your other hens are getting in those nests, doesn't sound like it is hard for them. I think your problem is to convince them to want to lay in the nests, not make them so they can get to them.
A photo showing where they are now laying in relationship to where you want them to lay could be quite useful. Is where they are laying that unacceptable? Could you put a nest there and accept that as their nest.
I don't think you are trying to solve the correct problem.