I have not had a pucker since switching to the Hobbs fusible batt. I lay the back on the old carpet in my basement, smooth the batt over it and then the top. I start pressing with a fairly hot dry iron in the middle and work outwards until the top is fused. I flip over the whole sandwich which is partially fused and start ironing the back from the middle out. There is always a little extra fabric that proceeds the iron and I work that out to the edges. When I have the back perfect, I put a few safety pins along the edges so that I don't accidentally peel the edge up.
I use knee pads when I iron on the floor. I sew all the sashing lines first with my walking foot (middle outwards)while the whole quilt is well fused. If needed I re-iron inside the squares before FMQ inside the squares. I am going to try the 505 spray soon on a quilt and see how that compares to my fusible batt because I am not getting any younger and crawling on the floor is getting harder. Good luck and just keep trying different things until you get something that works for you.