For what it's worth, I used the LFD Accessory plates (two of them), along with some long bolts/nuts/washers/etc to mount my X-Bull boards. The multitude of holes have a set that line up with the perfect width
My setup has 3/8-16x6" (these need to be replaced by 8" ones, but no where locally actually had them in stock) bolts, two per accessory plate, which are secured via washers on either side and a lockwasher/nut on the top side. The bolts then run through the existing mounting holes on the X-Bull boards. With the 6" bolts, they are long enough to run through 4 boards but not long enough to add wing nuts on top. Once I switch to the 8" bolts, I'll add two pairs of wing nuts, similar to LFD's hi-lift mount. For our last trip I just used two ratchet straps with the strap fed through the hand-holes on the boards and around the underside of the plates (and the bolts to hold them in place laterally/longitudinally).
This is the only photo I have of it that even slightly shows off how they are mounted... I can say it works wonders... Although, the day after using them to get out of some cement-like mud, we encountered rain... I was wondering why someone was pouring coffee down the windshield, alas it was just the mud running off the boards