Thanks. I started with the Pelican box
Carry Cases Plus - Hardigg Storm Cases, iM3300 Case but it was a tad narrow to hold my PETT toilet.
I had planned to put two boxes up there from the start so I bought a Boyt box
Carry Cases Plus - Boyt Tactical BT-H51 Waterproof Wheeled Hard Case for the second one. The Boyt is just large enough to hold the PETT.
Between the two cases I can stash a lot of light but bulky stuff up there including: PETT toilet, folding cot, sleeping bag, sleeping pad, extra shoes/dirty shoes, kindling wood, and more.
I had originally intended to buy something like a hardshell cargo box but everything out there is either too tall (16" or more) or too long to fit in the Baja rack. I even considered doing a stitch and glue box (like building a kayak) with fiberglass but shelved that as too much trouble for now.