1. Overbuild. A washer full of water is VERY heavy. Use screws, glue,
and metal bracing.

Nah. First off, a FL is NEVER "full of water." No where near it. (One
of the selling points is that they use water sparingly).

The pull out drawer idea is a good one. When pulling clothes out of
the washer, you may drop something - usually whites - and an open
drawer to catch same . . . well, it's a good thing. I got a
commercial base at Lowes for $25 (scratch and dent/closeout something
like that) and stuck it under the old dryer (doesn't fit right, but)
and it's work for a couple years now.

But I will build my own elevated double base when I move the units to
their new location. Try a Torsion Box approach to the top of the new
base. I'll bet one made of crossed half-lapped two by twos sandwiched
between half-inch plywood on top and quarter-inch in the bottom would
be more than sufficient for the 60-inch wide drawer approach. Use two
by twelves along the rear and on each end and Elmer's or similar wood
glue to bind all the members if the torsion top.

I've made similar torsion box "shelves" to hold large TV's using luan
5mm plywood and three quarter-inch white pine ribbs and it works
marvelously - very strong, stable and light weight.