Ohioguy writes:

Most lumber yards take returns.

I didn't get this at a lumber yard. These came from a thrift store
which has volunteers that go out and "reclaim" lumber from homes in
the area that are being torn down. Surprisingly, many of these 2x4's
are of superior quality to the new ones I've bought at Lowe's.
Anyway, they were just 90 cents each, and I felt good because using
them did not create any new demand for cut lumber.

Not surprising. 2x4s used to measure 2x4 and protected wood just gets
better with age.

If you are going to spend any time fabricating this, get the right
wood, pressure treated rated for ground contact.

Not really an option at this point - I already have the wood I want
to use, I can't take it back, and I need to get it out of the basement
to make room in the next couple of months. Since I also have a need
for some sort of border edging, I might as well just find out what the
best wood preservative I can use is. If I can't find anything else,
I'll just use a copper naphthenate solution.

I guess my question right now is - what chemical is soluble in an
oil based solution, with little or low solubility in water, and acts
as an insecticide, and possibly with fungicide qualities as well?
That is what I need.

My feeling is that it's a waste of time and money.
You can paint on a copper preservative but for the money spent,
time taken, I don't think it will add more than a year or two.