You can temporarily lock a trimpot with a tiny drop of silicone sealer. Then if you need to change the setting later on, you can peel the glue off. Do not use Locktite, it is too runny and will seep down inside the trimpot and contaminate the resistance element.
I gotta say, I'm in agreement with Temol. Unless you have a JFET with the right parameters, there may not be a correct trimpot setting. Sure you might be able to dial the drain voltage in to some "magic" value. Or you will find a setting that sounds good to you. But will it sound like an original production pedal? Probably not. Why do you suppose the pros don't use trimmers? It's because they know that the way to produce consistent results is to select JFETs with the correct parameters. In a production environment, they can buy them in large quantities and cherry-pick them. Most of us don't have that luxury. I've said it before, and I'll say it again: JFETs and Germanium BJTs are not plug-n-play. Having a particular part number is not a guarantee of success.
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