I don't think I'd even try to wear a holster under a tool belt, bro, especially in the humidity of MO in the summer. I was an industrial electrician for a good number of years, and before I made foreman I had the same issues you're asking about. I tried just about everything, and no matter what I did I ended up with a sore hip that was chapped raw.
If you're like me, changing sweat-filled shirts at the end of the day before you get in the truck is the norm. I couldn't STAND to get in my truck filled with sweat stink. I just kept my paddle holster folded up in my shirt, and would change in the porta John. When I came out to get in the truck and head home, no one was the wiser I was carrying.
Of course, I kept my piece on me at all times while working. I used an old multi-meter pouch with a velcro top on my tool belt, and it had a divider in it so the meter was on the bottom and the leads rolled up on top of the partition. I threw my 45 full size where the meter would go, closed the partition flap, and put some wire nuts on top of the partition so no one ever knew I was packing. Just looked like a pouch full of wire nuts, even though there were only about a dozen.
At the end of the day, grab my clean shirt with hoster rolled up inside it, enter the J john, remove my piece from the pouch and go concealed under my new shirt, and head home for a frosty one with my tool belt slung over my shoulder. In 10 years no one ever knew I was carrying and figured I didn't want to leave my tools layin around or drive home in a nasty shirt.
Wonder if them OSHA bastards would have been such peckerwoods if they knew I had 9 rounds of 45 on my belt?