I'm in the process of making a 'comfort seat' for my other half to use on the relatively rare occasions she will be riding with me.
I haven't fixed it yet. At the moment it's just photographed resting on top of my rack, so it's too early to be able to establish if it's been a success yet. Time will tell.
I saw this thread and thought I should show something about what i'm doing on this subject, so taken some quick snaps in case it inspires others to try something similar.
The plan i've been working toward all along with this bike was to initially make a sturdy rack/base for either my special homemade top box or soft-bags of various sizes, but then, when needed I could use the base to fix a larger pillion seat pad, that I think will be more comfortable than the stock pillion seat-pad.