I think if you can feel the ridges then it's not fitted  right. Yes, it is pain staking and does take time and experimentation.  It took me several tries to get it pretty much where I want it to be. I don't feel any ridges. I'm glad that you tried it and I agree, it's  not for everyone.

I'll accept that the possibility I didn't fit the stacked pieces of small tape correctly, creating the ridges that caused discomfort. Although I think I hit it correctly. I honestly experimented with a few different placements as suggested, but nothing felt natural. I think part of it is I go with a tighter thumbhole, which is going to make multiple stacked layers of small pieces of tape more noticeable.
 
Regardless, I'm glad it worked for you. One of the certainties of bowling is that there are always multiple ways of achieving something. I would hope others would experiment with the technique  and decide it works for them.

I have the majority of my balls drilled with thumbholes that are too big. I do this to allow for my fingers swelling since I wear a wrist brace, and sometimes a forearm brace as well.

Depending on the house my fingers will swell and I have to be able to get out the ball smoothly and quickly. I use white tape on the front of the hole for friction on my thumb pad. I use a piece of Velcro fabric for the backside to give myself a snug fit, that does not grab. It has worked well for me since the 90's.

When I first started bowling I actually used the hook side of Velcro, on the front side of the hole for friction on my thumb pads. Talk about building up a callous quick.....whew!

Magic Carpet would have made it easier on your thumb ...

I use 1" white tape in both the front and back of the thumb hole (I have a larger-than-average thumb), and I go back and forth using blue Hada patches on the outside of my thumb. I can see the logic in using white in front, black in back and not using skin tape and white tape (friction on friction), but as others have said, it's all about feel and personal preference. As such, I don't think there is a right or wrong way to use these products. If duct tape and magic carpet allow you to achieve a relaxed/comfortable/pressure free grip, I'm not going to tell you you're wrong for using it. Hell, I bowled with a guy who bowled with a missing finger insert for more than a full season because he was too cheap to get a replacement put in. I found that very foolish, but it didn't seem to bother him, and his bowling performance was still at the same level it had always been at. In the end, it's about doing you. If you aren't getting a good feel, experiment with the things that you see others doing. If you already have a good feel, don't let anyone make you feel like you're doing something wrong just because it's not the method they use.