To me the corner clamps make it much easier & I wouldn't try to build one without them. They shouldn't be hard to find & Home Depot will more then likely carry them. It's one of those things that u don't notice until u are looking for them. They also sell a bunch of different lengths of clamps all the way up to 3'. I have only built a couple, a 40 & 75 gallon. I clamped them exactly how Ron has his.
Like Ron mentioned, When clamping u have to be careful not to put to much pressure & create pressure points on the glass. I would get the glass set where it needs to be by hand & use the clamps to hold it in place. Don't use the clamps to get the glass where it needs to be, that's how u create pressure points.