I haven’t had any trouble with scratching. I turn the wheel very gently and if the wheel were to make heavy contact with the screw then the screw would simply give way and be moved aside. If the screws were fixed more securely then there would be a greater risk of the rim getting scraped but with the blu-tak there is more flexibility.
So far I’ve only used the screw set up on my Impulse wheel which I think is anodised. But I think it should work ok on a bare aluminium rim provided reasonable care is taken.
Alternatively you could use tooth picks etc., or, felt white board markers which would clearly mark the point of contact with the wheel. I just find the blu-tak and screws to be very accurate and handy.
btw jtrops your device looks to be very neat and accurate.
To be clear, I am currently building a bmx, so these are bike wheel. I hadn’t thought about unicycle bearings.
I tried the pen method, but I couldn’t get the pens to be stable enough on the unicycle frame, they kept moving. I’m sure if I were more industrious I could have rigged something up, but I didn’t mind spending the bucks to get one. It’s just one less thing for me to make and/or fix. That’s kind of “meta” to me. Building my own truing stand. But some people are into that kind of thing.
You’re probably right, that for a uni with either no brakes or disc brakes it’s not as crucial to get the wheel as true anyway. From what I have read it’s more crucial to get the spokes properly tensioned and as even as possible.
I actually seem to have a pretty good feel for when the spokes are properly tensioned. I have good musical tone, and I have enough well built wheel to compare against.