ordered my external over glass. did it all in 3/8" non-tempered plate glass with polished edges. ordered from laurel glass (they had the best pricing by $40). banner, dulles etc wanted around $104-175 and laurel quoted me $59. friendly service and hopefully they get the measurements right and polish the edges nicely.
i went with a 30" x 5.5" x 12" external overflow.
i wanted the 3/8" for rigidity on the bottom with the bulkheads and for the strength on the larger 30" glass in case i accidentally lean on it or put pressure on it, basically for peace of mind.
5.5" depth allows over an 1" on each end of the 2.5" bulkhead openings for drains of the bean animal. 30" give me plenty of room for acclimation, time-out space, etc and to water to dial in the bean animal.
will be using the same dow 795 black silicone i used on the internal overflow as it has proven VERY strong. i think even without bulkheads holding the internal to external boxes together the ~9g (70lbs) will hold nicely with just the dow product and thicker glass.