Ok, first don't laugh.
In the past I have used a hooked rug my husband did for my design wall for smaller projects - the yarn holds up the peices very well, even if I did not pin them. Currently I am actually using an old mattress that is leaning on the wall in my workroom - our back bedroom is undergoing a makeover and DH thought that right behind my sewing table was the perfect place to stow the mattresses.
Well I just started a DWR and found I was getting my mellon units mixed up. The hook rug is WAY too small for a project of this size. Out of frustration I just started pinning the finished units onto the mattress!! Works great!!! We have decided that we are getting a new bed for the room, so I think I will ask DH to salvage the mattress, cover it with a flannel sheet and attach it to the wall somehow.