Hi John, and all:
I'll add one more lower shelf option that has worked really well for me:
Screw narrow battens to the inside faces of your long lower stretchers and
then fill in the shelf space with short boards with tongue and groove
joints between them. This has several advantages. First, it was an
opportunity to use my matching T&G woodies. Always fun. Second, no glue or
fasteners for the actual shelf boards, making it easy to pull one out to
sweep out the shelf area. (I never do this, but it would be easy if I did!)
Finally, and I have done this: It's possible to remove just a portion of
the shelf to store something tall on the floor under the bench without
hindering holdfasts and such coming down from above.
Not sure any of that makes sense via my description, but this short-board
T&G shelf has been a real plus on my now 15-20 yr old bench.
Take care, all of you,