Listen to PeterParker. (That sounds about right when talking about Spidey)
A zipper up your back is a FAR inferior method of getting in and out of your Spidey suit. They may be called "Invisible zippers" but they are not actually invisible. They create a tiny gap. On a one color zentai suit, this usually isn't too obvious, but as soon as you have multiple colors and eye-drawing patterns, that gap becomes more obvious. The trick is to move that slight gap to the place where it will be least seen...and that just happens to be the sides and under the belt.
Don't try to reinvent the wheel here, many Spidey suit makers have already tried and vetted this process...and it's exactly what was used for the movie suits. It makes the entire back piece into a giant cat flap. Put your legs in, put your arms in and duck your head through, then zip up. You'll have people wondering how the hell you got into the suit. I've seen people convinced that someone in a Spider-Man costume must've been sewn into it.
Because, just like you guys are having a hard time imaging how someone would get in that way, no one will ever expect that method.
-Nick