Cargo Bag on OEM Tonneau Cover?

For my annual family vacation I need more storage room than the 5' bed of my Taco provides (I know this from several years experience with my 2005 TRD). I was considering buying a cap but I only need it once per year and it's a lot of coin to drop. That leads me to a stupid question: is it feasible/possible to mount a cargo bag on top of my OEM Toyota tonneau cover? Anyone ever try this or is this a dumb idea?

Thanks.

 

How about one of those trays that go into the receiver or get the rooftop racks with a pod? Seems to me that you would still want to acces the bed of your truck.

 

I have a hitch basket that I use constantly. Highly recommend it.

It still allows full use of my bed and the ability to open and close my tonneau cover.

 

How about a Roof rack and a bed extender?