That was the biggest flaw thatI saw in your plan... though it was good.
My basement sump utilizes a 55, but I learned the hard way...I broke it, cleaned it up, and out, bought a new panel, drilled a hole, and rebuilt it.
I think everyone needs that experience atleast once... like Luke Skywalker building his own Light Saber... your not a true reefer till you've cut your hands and sealed the cuts the same week with silicone.
I may know where there is a 55 with an end missing, but otherwise, the back banel on my 55 costed around 35-40$ undrilled.
good luck ordering a custom one, I tried that too. You may checkout Glasscages.com, and look at the acrylic ones. Those you can drill. I think they want to have it shipped to a biz address though, pm me if you don't have one, don't think they will ship to residentials...don't quote me on that...
call them.
J