Please have a look at the links at the bottom of this page
http://www.bobgolds.com/AbsorptionCoefficients.htm
starting from Jon Risch's Absorbers to Vshine's Wall Absorbers, for some ideas.
If you use a cloth covering that you can breath through (i.e. wrap it around your head and not suffocate) then it will not change the absorbtion coefficeients much if at all. Speaker cloth certainly falls into that category. So does chicken wire, and burlap.
Cloth that you can't breath through is less predictable. But generally speaking it'll probably reflect some higher frequencies, but let low frequencies through. Garbage Bags certainly fall into this category, as does felt.
You can glue cloth with 3M Super 77 spray, but any place that you've got glue, especially thick glue, that fills the pores of the cloth, will change it's absorbtion coefficients, as well as change it's stretch/appearance. So I'm thinking that you would wrap the 703 like a christmas present and glue only on the back. Alternatively you can sew the cover like a pillow case.