We dont hunt portable blinds. We do have 4 blinds that are permanent that we use(made of wood) that are on public land, they are basicly the same size as what you have. Generally we camo the outside with cedar branches and stick some bullrushes in all over.
I have 3 pieces of 1/2 steel wire that are about 5' long and are in a semi circle shape. we push the ends of these rods in to the blind at both ands and the middle,, it forms a dome shape over the blind.
We then throw a camo material over top and staple itat both ends, we then throw alittle bit of natural vegetation on the top. this way it covers the top of the blind and can be pushed forward or backwards depending on bird approach. We take the steel rods with us when we leave,, this way if someone else decides too use the blind they better bring some sort of roof with them or theyre going too have a hard time getting birds too commit. couldnt tell you how many times weve gone back the next day after a good shoot and someone else has watched us tear them up and they are in that blind next day and dont fire a shot.
this is the material we use its that die cut stuff,, this stuff is great because you can see out of it without having too lift a corner.
http://www.cabelas.com/cabelas/en/templates/pod/vertical-pod3.jsp?_DARGS=/cabelas/en/common/catalog/pod-link.jsp_A&_DAV=MainCatcat20712&rid=&i ndexId=cat601993&navAction=push&navCount=1 &parentType=index&parentId=cat601993&i d=0005729