Hi Seany
I'm feeling your pain mate, I had exactly the same problem myself a few months ago. I've got the ST3 too and really struggled to find one that fit properly.
In the end I gave up and just went for the seat I would have gone for had I not had nice leather Recaros
.
It doesn't fit properly but it's dead easy to take in and out as and when I need it and really lightweight.. (I do only use mine one or two days a week though) so it just bounces around in the boot until I need it. I just pop a towel or blanket down underneath it when i put it in and that helps stop the seat being marked.
I know it's not the perfect solution but i just ended up resorting to it anyway!
I'm feeling your pain mate, I had exactly the same problem myself a few months ago. I've got the ST3 too and really struggled to find one that fit properly.
In the end I gave up and just went for the seat I would have gone for had I not had nice leather Recaros

It doesn't fit properly but it's dead easy to take in and out as and when I need it and really lightweight.. (I do only use mine one or two days a week though) so it just bounces around in the boot until I need it. I just pop a towel or blanket down underneath it when i put it in and that helps stop the seat being marked.
I know it's not the perfect solution but i just ended up resorting to it anyway!