Here's my thoughts:
I think you DONT want a trailer that has a crossmember below the CB slot. If
you do, then you've just eliminated the option of ever removing the board
while the boat is on the trailer (say at a service station lift) so you'll
be stuck going to the boatyard and paying boatyard prices.
Plus, lowering the centerboard on the trailer will increase setup time at
the ramp, and I'm SURE I'd forget a time or two and make a bloody fool of
myself at the ramp.
My CB has two 3/8 holes for securing it in the up position. I run bolts
through there with nylock nuts and I tighten them. Faster would be some
hitch pins secured by cotters (not sure of the terminology - I'm talking
about the way a trailer tongue is held in a receiver hitch). One way or
another, you MUST secure the pins/bolts in there, otherwise the will rattle
out while trailering.
I also put enough tension on the cable to where it gives some support too,
but the CB is definitely resting on the pins, so the load is shared by two
large bolts and the cable.
I've thought about adding a removable wooden crossmember beneath the CB slot
on the trailer as a backup incase the above 3 things fail.