I need one too. Mine would also need to fit the xtra long flaps of the Stubben. The saddle came with the one, but I want to keep it nice incase I ever sell the saddle. NO - I don't sew so following the instructions off a website won't work.
There are some patterns to sew your own cover, should you be inclined. And are not too complicated of a project. That way you can pick out material, color, etc. Also, a local seamstress/taylor might do the job for a reasonable price.
Dover Saddlery also has some zip-type, lined dressage bags. I bet if you called their warehouse, someone could get the specific measurements of the dressage bag for you.
“A horse is the projection of peoples' dreams about themselves - strong, powerful, beautiful - and it has the capability of giving us escape from our mundane existence.” ~Pam Brown
I would contact SmartPak. I was just speaking with someone there about the fact that everybody (not really, but had to make a point) rides in an 18" drssage saddle and it's hard to find covers and special pads. I needed one of those lift front pads for Emily as she is going through a little growth spurt and a Mattes correction pad is not in my budget. Someone actually went into the warehouse and measured the different pads for me. That is what I call customer service. Call SmartPak and ask for a manager. I am sure they can get something made for you. They are fantastic!