This is what I do; undo all that you have done.
Then spray starch the back real good and the
the top also, then tape the backing down on the
floor again, spray with basting spray real good, lay
the batting down, then spray baste on top of it, then
lay the starched top on that. Smooth out as you go
if needed. You will not need any pins at all.
These layers will stay together without pins.
I think you will like this way very much.