When I first started machine quilting whole quilts, I used a fusible batting which you iron to the back and front. This kept it all together enough for me to feel comfortable with the bulk. You might try a couple of card tables to the side and back of your table to hold it, too. Even an ironing board helps extend the space and hold the bulk. Now I simply run my quilt up against the wall behind my machine. Do whatever works for you!
I have found that using unwashed backing fabric is stiffer and helps me avoid puckers. I also quilt in sections - Marti Michell has a book with that title - sometimes quilting just a third of the quilt at a time and then sewing the sections together. I have done a couple of king- and queen-sized quilts that way.
Keep at it - you will find what works for you.