Is this enough to keep it dry inside
imho, if the cover is serviceable (not wore out), i think it can be.
If you want a good cover you need to spend the $$$ for a custom made cover.
this is likely true. but me personally, i've never had that much money. so i've stayed with aftermarket generic covers.
you need to have braces or supports or what ever you want to call them under the cover to prop it up and support it. Without, water and/or snow will pool on the cover and leak into the boat.
this is for sure true.
i've had 5 or six covers for my boat since i got it 12 years ago. i started with the 300-400 dollar type generic covers. they lasted from about 3-4 years. the last of those years of service, the covers were degraded and didn't work well, ie. leaked.
i've since gone to $100 covers from menards. i'm getting 2-3 years out of these covers.
as far as fit goes. i'm using a 21' cover on a 19' boat. reason for this is i have a hardtop which makes my need for wide beam necessary.
imho, the statement that you need support poles is critical. you have to make a tent out of the cover. no low spots at all. however you achieve this is up to you.
i also remove the snow off my cover right after each snow. if it's gonna be ice, i do use two covers. though not a tarp. i use the last non serviceable cover i have on hand. after the ice storm, i'll remove the top cover leaving behind my best cover i'm using to protect my boat.
in conclusion, i'm fine with aftermarket covers that are serviceable. you just need to support it proper (most important). you need to do some care over the winter to keep the cover clean, clear of snow and ice. i've had good results with this method. i do try hard to keep my 43 year old boat (that is pretty darn nice inside and out) in good shape. so i want to keep it this way. though i want to do it as cheap as possible as i'm not a rich boater. and i think it can be done with a cheaper cover.