Angelfishz from your tank dimensions, your glass thickeness is about 5~6mm meant for a freshwater setup? Correct me if i'm wrong.
To custom make a glass overflow box it'll stress your main tank glass further as its supported on the glass! 5-6mm glass thickness will not support an overflow box made of glass! its too heavy! I suggest using arcylic, its lighter.
Sorry bro Ducados no hard feelings but its not so simple as to get glass cut and silicon it up. There will be alot of trial and error to get the measurement correct plus correct flow which will cost more. If angelfishz were to get off hand ready made overflow box it will solve alot of problems.
Here is a link that will give you some idea of what we're talking about.
http://aqua.span.com.sg/?cat=1&paged=2
http://www.iaquatic.com/
There is also the in built skimmer save you on a sump tank. They can be placed inside your tank and its cheaper. Why i say its cheaper....sump tank 33x12x12 est $50 + overflow box est $150 + return pump est $30 = $230 which i think if you get a in built tank skimmer its cheaper.
Look for skimmer, tunze doc skimmer 9002 your tank size is 50 US gallons
http://www.reefdepot.com.sg/
More detail product info below.
http://www.tunze.com/149.html?&L=1&C=SG&user_tunzeprod_pi1[predid]=-infoxunter050
So its your call dude. The above price is just an estimate don't quote me that this is the price.