Yes, this is for a manufactured aquarium, I don’t have the DIY skills to make anything myself unfortunately.If I understand correctly... The issue is, he is buying a manufactured aquarium. And beyond 120" the price increases are quite high.
If this were a DIY tank, extending the wooden box would be a nominal cost, probably less than whatever other filtration system is going to add.
The same option is still available with a manufactured aquarium, though it would be costly, less efficient and (imo) a pain in the rear to install. The space on each end is roughly 12"x48"@30" deep, the same as a 110 Gal Aquarium. slide one in each end slot, drill as necessary and plumb them in as filters. Instead of 110 Gal aquariums you could consider some other container of roughly similar dimensions. Since it doesn't need to be see through your options really open up. Some of the suggestions above are in the same ballpark as this.
I’ll inquire to see what the extra cost is to lengthen the tank just to see, although I do expect it to be cost prohibitive.