Inflatables are of two type. Single-walled with interior pressure, and double walled with the interior at ambient pressure. I presume you are going for double-walled.
The shape can be easy.
Antfarm used an electric iron with the material drapped over a thin edge:
https://www.foundsf.org/images/9/98/...inflatable.jpg
The hard part is attachment and ingress/egress. (I'm not retired, I just between gigs)