Water height can be adjusted a small amount by the return pump flow rate. Using a router bit and a straight edge clamped to the box will most likely give you the best results. In addition to a straight edge to guide the path of the router another one, or some kind of stop, to control the length of the slot.
I'd second Ron in regards to drilling holes rather than trying to extend the length of the weir slots themselves. Not a lot of margin for error there and not something i'd recommend trying to freehand with a router.
Thanks a lot everyone who replied. I went through them all. It seems like the general consensus is that dremel wont look good and the best way to do this is with a router and a jig. OR I can drill just simply holes with a drill bit for acrylic. I am leaning more towards the drill as I have a small fear of routers. I've never used one but I want to learn. Not sure if I want this first projectSo I recommend checking out something called a fluted drill bit. This is a company known as Tap Plastics.
Regular drill bits are not the greatest option. They will work, I use them all the time for drilling holes in acrylic. But there is the possibility of cracking the plastic.
Fluted bits, allow the plastic that is being cut to escape upward, preventing any additional pressure being applied to your work piece. It also leaves a rather clean cut.
That being said, I have never seen them at Lowe’s or Home Depot. So you would have to order them somewhere.
Both Acrylic and Polycarbonate are "grabby". You are going to split it drilling holes that close to an edge. Save yourself the headache and just order the proper overflow box. The router, jig setup and time is going to cost you more than just replacing it.
Modular marine replied and gave me a $120 for a new box with longer weirs and a little bit shorter top. I will go ahead and order that...
I think I will do both actually, I will order the modular marine replacement for the perfect clean look. I will also satisfy the DIYer in me and try to make bigger slots. If I mess them up Ill just replace overflow box with new when one