I looked on ebay. Does it have to be a unidrum, not a beseler or some other? I'm curious how you plan to get 4x5 negatives secured in an 8x10 drum. You're going to develop your BxW 4x5 negatives in a color print drum? I'd like to hear more on this.
This article has more information than what I could provide. http://www.largeformatphotography.info/unicolor/. Once I get up and going I can let you know how it goes.
If you find one without the neg separators, you can use small rings made out of PEX (flexible plumbing tube). Cut a ring about 1/4" wide with a coping saw, and then make a split in one side. The PEX has some nice flex and will fit over the V channel on the inside of the drum.
