That put us on the search for a "transport" chair for around the house; he is getting quite heavy to carry from room to room (and if you have ever tried to lift 32 pounds of "dead weight" you know it can be brutal to do all the time), so we needed something to ease our back strain. And he needed something more supportive than a stroller. We also wanted it to be big enough to grow with him, and to have big wheels that he could roll himself if he decides he wants to push himself as he gets older. However, it isn't time for Mason to get a new custom wheelchair (due to insurance requirements) so we had to improvise in order to get him something both that met the above requirements, and especially was affordable.
What we ended up with is this chair--Excel Kidz Pediatric Wheelchair.
It is about the smallest "standard issue" type pediatric chair we could find online. (I found one smaller but it was out of stock everywhere). It was too big (14" wide by 12" deep) for Mason as is, so it will grow with him; it has telescoping handles which we love (although one handle wouldn't stay in the up position when it arrived - we had to screw it up manually using a drill).
So...while this chair met Mason's need for light, portable wheelchair to use in the house, it didn't have much support. I had already gotten a contoured cushion for the seat but he still had too much room on each side.
I looked online for back and lateral cushions to add but couldn't find any that would fit his kid-sized wheelchair; and all the other sizes cost way too much anyway.
So I ended up using an old zip-up black hoodie upside down with the sleeves filled with towel rolls as lateral supports and a piece of foam and towel rolls inside as a back cushion.