Ideas for a sump...

[biggerstretch]

HOB overflows are OK but not the best for piece of mind. I built a few in the past and they worked however I never really trusted them.

I would shut the tank down by moving it into another aquarium or something and get the back glass drilled. Back glass is generally is not tempered so it can be drilled. Put a 90 pipe fitting in the tank to a flange and then run the pipe down to the sump. As far as the sump goes , building some baffles and filter stands is simple. The smaller tank in the bigger one works well also. Never liked the Tupperware sumps as they tend to be too flexible ( as stated previously )

The solution to pollution is dilution , the bigger the sump the better however with a smaller tank it isn't practical to have an 80 gallon sump. I suppose a 5 gallon pail would work as a sump however we all want to have our tanks and setups look nice...
If you have lots of cash get the sump custom made from acrylic, if you are on a budget then see what kind of a used tank you can find and either silicone in some baffles or put another smaller tank inside it. I however do not trust HOB overflows, to me it is just a way of not doing it right in the first place....