I have a 125 tank as well, a Perfecto to be honest. I wanted a sump that would fit under my tank and within the parameters of my stand. I ended up getting a custom 33 gallon glass tank made for around $60.00. I ordered it from a nearby fish store. although my dims were roughly 34L x 15W x 17T. I wish I would have made it a little shorter for when I need to get in there. I searched all over creation for a 15W but came up empty. Don't spend the $200 - $300 for a custom acrylic, it just isn't worth it for a sump.
Custom acrylic can be had for next to nothing if you're not worried about cosmetics. My LFS does some custom work on the side, and I had him order a sheet of 1/4" acrylic for me. We built a 5'x1'x1' sump on his back porch in less than an hour. Total cost? $50 for material.
