Sandwiching is always a chore. I have experimented with the floor, design wall, bed. I think I may have a handy and inexpensive help mate here.
Picture two is a close up of pipe insulation, not spendy at all and readily available. I'm showing the line that you would break open to place around an exposed pipe. This a great guide for lining up the top/batting or backing. Picture one is the garden stake that I insert to stabilize. You can add more length by simply taping more on and adding another stake. For storage I remove the stakes and roll up the insulation. Picture 3 is the part I like I can pin to this service, great stability, it also shows the water soluble mark for the center. I find and mark the center of batting, both sides and do the same for the top and back. Picture 4 is how I apply the glue in a wave line rather than straight across, but you could bead or spray baste. I prefer the glue as I always over do it with the spray and it gums up the needle. The last picture is rolling it out lining up my marks.