I think that I used 992 out of a pack of 1000. 3/8 inch, and an electric stapler worked fine for me.
OP is not using a sewing machine, or mesh along the bottom of the bolster, but a couple of tips for anyone using this as a reference thread in the future.
Phifertex does not stretch like vinyldoes, so you can very easily end up with a curve on the bottom edge, especially if you are sewing across the roll and pleat.
I learned this on the second bolster, after cutting the first along the factory edge, where the pleats do not go all the way to the edge. In the future, I would cut one bolster from each edge, instead of two side-by-side, and sew the Phifertex 1/4 of an inch from where the roll and pleat starts. Cut away that inch or so of foam underneath.
If you are using mesh, staple the bottom edge of the bolster first, to keep that seam from creeping around the edge in places along the bottom, and to keep your mesh a consistent width.
If your bolster is going to be 2 inches thick, draw a line along the mesh an inch and a half from the seam, and staple that line to the edge of your board.
If your bolster has a 90° inside curve, like the Seafarer, dead center of that curve is your starting point for stapling. Get that curve right, and tight, stapled top and bottom, and the rest is a breeze. I can’t tell you how to do that corner without a sewing machine. I was going to eliminate the curve, and do a two piece bolster, if I did not figure out the seam on some scrap vinyl. Trying to do that original insert to make the curve can cost you a lot of expensive vinyl if you don’t get it right the first time.