Edit (6/5/19):
Made a new version using Fusion 360. Changed the design quite a bit to make it sleeker and hollow to potentially carry small items like your phone and Con badge.

Not sure if it will print right, but check it out here:
https://www.thingiverse.com/thing:3673277

Edit (5/31/19):
Printed the file finally. It was not flat on the bottom all the way (sorry still a beginner at modeling). I made it flat by lowering it 4mm below the print bed. This fixed the level of the back and allowed it to adhere to the print bed much easier.

There are weird lines all throughout the model. I am not sure if that has to do with the size I modeled it in blender or if there is some function to remove the lines and make things smooth. Any suggestions here would be appreciated. I did not expect these textures and don't know how to fix them. I will just fix it when I move into the finishing stage for this prop, but changed this file to a WIP until someone is able to fix it.

I tested the size by holding it between my shoulders and looks about right when comparing to several comic panes (as the drawing are never really consistent). This one was printed with a width of 23mm.

I then attached magnets to the back and tested it through my inexpensive Party City Spider-Man suit. It stays in place well. (Ignore the spider symbol on the suit as Superior Spider-Man Suit does not have that printed on the back and my actual suit is still on order). The suit I ordered has a U-Shaped zipper so the zipper will also not be in the way. It was pretty easy with the strength of the magnets to simply put them under the suit, hold the backpack close, and let the magnets align themselves under the suit. I was then able to just slide it into place and have my girlfriend help straighten it up.

The little spider bot on my shoulder is another magnet test. More pictures can be found on my make: https://www.thingiverse.com/make:639943

The original spider bot file can be found here:
https://www.thingiverse.com/thing:2991617

This is the first thing I ever 3D modeled. I made it in Blender after messing around in the software for a couple hours. It is my own version based off of combining a couple comic panels (in the pics) from the Superior Spider-Man series.

I might try and make some recessed areas in the back for some strong magnets to attach to a Spider-Man suit without having to use straps or velcro. I printed a crappy looking one at 5% infill with .3mm layer height and it was super lightweight. Thinking magnets would be doable...

Please, by all means.... make a better version for me to print. I have no clue what I am doing and would love a slick version for cosplay. I added the Blender file into the download if you want to mess with it.

Like I said... first 3D model, don't know what I am doing... be nice. ;)

Apologies in advance. I wasn't sure how to get the scale right in blender. I would recommend Scaling it to ~230 to 250mm wide depending on your shoulder width and how you want it to sit.