A couple of points to consider; here is an edited exurb from another thread ("Shrink Wrap");
Shrink wrap gets stretched tight and when put on properly, most of the snow slides off leaving little snow load and chance of structural collapse to worry about. If done right, it won’t get caught by the wind and damage gel. Because you can tightly wrap without leaving any openings, it also keeps the critters out. An optional zippered door is great for inside access while still maintaining a tight fit.
While a custom made cover will go on tight, it's unlikely you'll get it as tight as shrink wrap so some pockets of snow will accumulate over time and increase the chance of tearing due to snow load. Too tight and the stitching will let go over time. Wind is your enemy, if it's not a good fit, it'll catch the wind. Once the cover starts blowing and flapping around, it’s just a matter of time before it flies apart. There would be nothing worse than you having to redo it during a cold windy winter.

While it’s blowing around, it’s slapping your gel. I’ve never seen serious damage from this, but it could get bad enough to rub the gel such that you’re left with a cloudy surface that will need to be polished out. If it is to be tight enough to shed snow, the framing must be the exact same layout and dimensions every year or you won’t get the intended fit. If you use the same frame every year, you'll need to consider storage for the cover and framing. If the build doesn't incorporate a frame, it will sag and collect snow.
Also consider that if the shrink wrap fails in the middle of winter, it's the marina's or provider's responsibility to repair it at their expense in the middle of winter. If you put up your own cover and it fails, you're responsible for the fix (in the middle of winter!), and if it blows away in pieces, and ends up on someone's property or in the water, you're responsible for the clean up.
I've seen some good custom covers, none of them tight enough to shed snow in some areas. I've also seen several covers fail. Poor shrink wrap jobs fail also. I’ve never done a cost analysis on a custom cover vs shrink wrap over a given time frame as the longevity of a custom cover really depends on the material used and environmental factors. The cost of shrink wrapping up my way though makes the custom cover a very attractive option, but if the provider says it'll last 10 years, I'm only expecting it to last 5, and factoring in regular maintenance such as stitching, patching, water proofing if desired.