Powerchair Seating Problems

How many of you have difficulty finding the correct wheelchair seating for your needs? I've been through foam backs which made me sink into the backrest, and hard backs (Invacare Matrx and Sunrise Medical Jay3) which is too hard and uncomfortable for me. I tend to slip and slide in my seat despite having a deep contour back and it being shoulder height. Is there anyone here who has, or has tried, custom-made seating? I can't seem to manage to be comfortable and stable with other kinds. I emailed the Sunrise Medical Built For Me customisation team and the HQ, and they say there isn't a different type of material to go inside a Jay backrest. I have spine align wedges and the largest positioning wedges available for the Jay 3 seating system (I wasn't sure if I did or not, but my partner measured them and I do) they're 5cm high and they're inside my cushion, but it doesn't seem to make a blind bit of difference to my positioning and comfort. I need to replace my seating, but am at a loss as to what type to go for. Advice? I don't want to buy another hard aluminium back- there isn't any other that will be high enough for me in the Jay 3 range of backs or the Invacare one. I also don't want to waste my money on something that won't be ok in the long run, although the only way to know how it feels is to try it. Most of the technicians have given me 15 minutes to try out the feeling of a cushion or back, and the thing is it feels fine at the beginning because it's a different product, but I only notice how it really feels for me with time and use. Due to my posture, I get back, rib and neck pain, and slip so much that after even a short time in my chair I'm almost bent double. This absolutely is not great, given that I have to use the chair all day every day.
I have a hip/ pelvis belt and also Velcro fastenings on my footplates to keep my feet in place, The effort of trying to sit in my current seating just exhausts me to the point where I have more and more spasms and can't move at all