I drilled thru the back panel. Consider going with larger holes than normal if possible. The extra fittings may restrict the flow somewhat. If You feel comfortable with DIY, it's not too difficult. However that 1st hole can be unnerving! I also ordered a diamond holesaw & practiced same as Searoyal.
You may want to think about one large center overflow. I would also like to hear what the experts have to say about the pros & cons of two corner overflows compared to one big center one.
The biggest consideration in overflows IMO should be how much surface area it can skim. You want to maximize the linear amount of surface skimming.
Usually two corner overflows will have more linear skimming surfaces than a single center overflow.
Ideal setup is a coast to coast external overflow that skims the full length of the tank. But that is not practical in most cases.
Finally AFA drilling the holes yourself. I practiced doing it with a carbide hole saw on smaller tanks. My DIY skill is pretty low so I just save it for the glass shops now.
But what I did was use some plumbers putty to build up a wall around the area I was drilling and got a couple bicycle water bottles (few bucks at the bike shop). I built a small stand to hold my tank up under the drill press, and slowly started drilling, keep using the water bottle to squirt water into where the bit is drilling. The water will keep it submerged, and the squirt bottle kinda helps keep it cleaned out.
I personally would do it without a drill press or some kind of device that will hold the drill perfectly perpendicular to the glass pane.