Thanks for the replys and info. Sounds like it might be best to just save another $100 and get another 26" cabinet and top box combo....then I will need a few more tools to fill it up!
My initial searching did not answer this question...hopefully someone sees this and has tried it:
I was thinking of adding the US General (Harbor Freight #68785) side box to my 26" cabinet (#67831).
One of the reviews on the Harbor Freight website says it makes the whole box "tippy". Maybe a...