michael writes:

Noticed the
boards on the trailer distorting under loading from binders. A couple
4x4s lagged or thru-bolted to the base spreads the load out nicely,
and enhances stability.

The photos from above give the appearance of the boards bending, but
actually it was one of them being previously warped from weathering. If
you look from the side you see that they weren't bending much from the
load.

I would have bolted right through the floorboards, using some lengths of
SuperStrut underneath to spread the load across, if I had known the rental
people would have allowed drilling the floorboards, which I didn't. If you
mean to bolt the machine on top of 4x4 lumber, then that adds another
failure possibility if the wood splits from a bump with all that mass
concentrated on the thin bottom edge of the base, so you have a trade off.