Yeah, at least around here (tropical rainforest Maryland) those pallets would turn into slug convention centers in no time. Believe me,I know.

Been there, done that. I once saw in a community garden in the next county where they put down old carpet as "mulch" between the rows of what was supposed to be an herb garden. Thought this was a great idea. Not! The weeds still came up between the carpet and it was hellaciously difficult to try to pull the damned things up when it was obvious that the idea was a monumental failure. Similarly, using some of the "garden weed cloth" that's sold in local stores had the same problem. The cloth ripped to shreds when the weeds came up through it, even though there was mulch on top of it, and some of the "mulch" I used was slabs of concrete. I finally found some professional 20-year nursery weed cloth that has done wonders and I didn't even need to put any type of mulch over it. Not cheap, but definitely worth the money.

I bought a 2' deep 6 foot tank to plant in- could not find an old tank. Our yard down in Savannah is very low and floods in the late summer monsoons thus I plant in raised beds and containers.

The farm store I bought stuff at has several with various plantings in them. What they told me to do was fill the bottom foot with rocks, then soil on top, and just leave the drain plug out thus I did not have to drill a new tank full of holes lest I decide I need it to bathe in one day or actually water livestock.