Wheelchair position

Just looking to see what types of backrests/cushions/positioning any of you have done to help support the lower back and minimise slumped position while in your wheelchair? (I'm T4 complete for context)

I'm based in the UK and have been to wheelchair services on multiple occasions but they never seem to get it right (often making it worse) and with over a 6 month wait to be seen to correct the work they've done, I figured I'd ask you guys first so I can try and tell them how they might approach this.

I've attached some photos to help explain what I mean. I've got a slightly bucketed seat and a contour the bottom of the backrest for me to scoot back into currently which makes it look like I'm slipping down but I promise I'm not!

Also don't mind the dog lol, he's clearly having a crisis😂 Tia everyone :)