Are you planning to machine quilt it? If so, a sheet should be fine on the back. For making the quilt sandwich I will tell you how I do mine. I spread out the backing on the old carpet on my basement floor. I smooth out a Hobbs 80/20 fusible quilt batt on the backing. I smooth out the top and starting in the middle of the sandwich, I iron outwards. Once the whole front is fused I flip it over and do the back.
I have only done one quilt with 505 sorry basting and it tuned out well also.
There are many ways to do the sandwich and you will need to find the method that works for you. There is safety pin basting, thread basting, pin moor basting etc. etc. Some people bast on a table, some on the floor, some use boards like Sharon Schamber....