I picked up a silicone bread loaf pan from the the thrift store and cut the smooth outer edge off (the straighter and longer the edge the better… maybe a cake pan?). I found this worked perfectly for welding seams as it has a slight give to it when you press with the iron and you don’t end up with nearly as many voids in the weld as you do when working on a hard surface. It is also fine with high heat, doesn’t stick and the underside of the weld is smooth and unblemished.
I created a design and then drew it on the back of a sheet of light coloured dressmakers carbon paper. I put the carbon paper design over a suitable scrap of TPU fabric and traced my design… transferring the image on to the fabric. Then I cut out the design and heat sealed it onto the hull. The transfer lines wash off.