I have a couple of Hercules guitar stands. The advertising blurb says the rubber padding does not react with nitro varnish finishes unlike some of the cheaper stands where the rubber eventually ruins the varnish. Hercules do multi guitar stands too.
I think if you are making custom diy stands using really quality timber then there has to be a better solution than pipe insulation. That will look terrible! 
The type of inert rubber that Hercules uses must be available to buy.
Edit I’ve been looking online and haven’t yet found inert rubber.
I did however find this excerpt from a guitar magazine.
1) Nitrocellulose and Possible Chemical Reactions are Unpredictable
There are a lot of companies out there that claim their products and stands are nitrocellulose safe, however, you never know what can happen.
I remember reading the statement from one reputable brand who said it was impossible for them to say their stands would be 100% reliable whilst holding nitrocellulose instruments.
Simply put, it might be wise to stitch on a piece of towel behind the neck to keep your guitar finish safe, rather than trust the marketing and advertising of a brand that wants your money and really won’t care if the finish on your neck is ruined.
Good luck Robert. There must be a solution to your issue. I somehow don’t think people like Joe Bonamassa who own extremely expensive vintage guitars will mess about with old socks over their stands!