Could you describe the draft of the towel that you hope to recreate?
I know zip about RH and was going to recommend that Gist site as well. They have converted a number of their patterns from RH to shafted loom.
As for thread conversion, what is the EPI in 3/2 that your draft states?
8/2will be 18-24 epi as recommended in Jane's master sett chart as well as in handwoven.
softer to firmer. You'll have to add threads to your draft unless you want a much narrower towel.
For ex: 3/2 at 15 epi for 10" width you would need 150 epi. if you threaded 150 ends in 8/2 cotton 20 epi you would have a 7.5" width on the loom. If you wanted a 10" wide 8/2 cotton towel, you need 200 end.
Drawing it out on paper and doing some maths really helps. There is a PDF for project planning that is helpful.
Hope this makes sense.