Cable routing help

So I love my 800D case save for one small part, along the back side where the MB tray and some bottom shelf meet, there is a half inch lip that sticks out and severely kills the ability to route cables cleanly, especially fat rounded ones like my PSU has. In trying to figure out what to do about this, I've pondered getting a new case (partially too because the 800D is a little banged up), but realized after a rebuild of my system, that the PSU cables would be an issue with probably most cases. So I would like to know how I can make cut outs in this section to provide better space to fit my fat cables. Quick bit of info, I do not have access to a dremmel, and would like this to be as cost effective as possible a MOD, meaning, I'd rather not have to buy a dremmel, if possible to do with some heavy duty metal cutters, but I will need to buy a tool probably. Pic of what what I am talking about provided. Thanks!

First image is just a shot of the back side of the case, 2nd has the red line indicating the lip (don't mind my shaky hand for making the line).

 

Could be done with a hacksaw(more low-tech and affordable). I'd also get some window trim, to cover where you made your cut(s), like how the holes that are pre-cut into the case are filled with rubber, window trim will prevent the cables rubbing against the bare metal and wearing into the cable or shorting out.

 

Forget your cable management issues; Fix your dust problem first!