Here's my experiences:
Universal Fit = Usually says something like "fits 20-22ft boats" - Probably won't fit very well at all.
Semi-Custom Fit = Usually says "fits Mariah" - fits better than a universal, but still not very well.
Custom Fit = Will usually say something like "Fits Mariah 210 Shabah" Made with the measurements from the manufacturer, fits better, but still not perfect.
Custom Made Cover = Best fit. This requires contacting a local canvas shop to take the measurements, and have them custom make one specifically for your boat. This will be an exact fit, and may cost a bit more, but it's your best option.
You get what you pay for with tops. Take it from someone who's bought a few in my life.

I ended up just having a custom cockpit cover custom made, and it's perfect. I trailer hundreds of miles a year with it, and haven't had a single issue. Plus it looks pretty sweet too, much better than the original vinyl cockpit cover. I use my "semi-custom" for a winter cover since it doesn't fit very well.