You are right about the gas springs not being constant force.
I found off the shelf springs that gain about 20% in force over the entire stroke of the spring. You could get a longer spring than needed. Say 6" if you only need 3" of stroke. Size the spring 10% under your load. Then the spring would balance perfectly at the bottom of stroke and lift 90% at the top.
I noticed on this website
http://www.indgassprings.com
that you can get special springs that only gain 4% force over the stroke length.
I use these springs as counterbalances on doors, lifting tables, and they are very easy to install and work well.