I saw you had a link to Steady Garage in your first post, but I couldn't find the seat on their website.
Do you have a link that will take me directly to the seat?
I emailed them directly as coincidentally enough another member posted they were in contact with someone there who mentioned they'd be doing seats soon. I simply went to their help and opened a support ticket asking if they would sell me one direct
I attached the pic from that thread if you want to see the options available.