Buggy liner / cushion to prevent slumping?

[Beetleandbug]

Does such a thing exist? My DS is 8 months old and is able to sit unaided but always slumps to the sides in my Uppababy Vista.

I've tried adjusting the straps to the tightest setting but it still happens. It can't be good for his back? He gets very frustrated if I keep the seat reclined (and because it's a bucket seat his feet are then in the air)

I was wondering if anyone could recommend a support liner to keep him straight?

I got a Buggysnuggle for my Vista and it stopped DD slipping down. She's fine without it now (22 months) but she always used to slump in the pushchair without it on when she was smaller.

uppababy do a "snugseat "

I'll look that up Llama, thanks.

Crazy, I do have the newborn snugseat but DS has long outgrown the head rest and it just lines the seat rather than providing any support at the sides. It's a very wide seat and I think that's the problem.

I used the John lewis lamb wool, fits well and keeps them warm in winter, cool in summer