Here you go - this is easy. If you can drill a hole, this'll work. I'll use my truck and some X-Bulls I've got to demonstrate what I did. First measure the distance between the mounting holes on your board at each end, and see if you can space the crossbars on your roof the same distance. Mine were 24" apart on center on the inside holes, so I was good. Mine also have holes on the very ends, which would have made the distance about 40" apart.
I used 2" long fully threaded 1/4 bolts, and covered them with 1/4 " ID vacuum hose. 1/4" ID vacuum line rubber caps would make a neater finish (if you do, get tall vacuum caps; they can always be cut shorter). There's only about 1" of threaded bolt showing above the nut in my set-up. If your crossbars are thicker, you might want a little longer bolt (or just have less thread showing above the crossbar - as long as it captures the bottom board like in the fifth picture, you're good). The upside down U-bolt is optional. The second picture shows with and without the rubber vacuum line.
Measure twice, drill once. Get the boards and shovel exactly where you want them (front-to-back, & side-to-side), and then mark the crossbars with a sharpie. Remove crossbars and drill holes, mount nuts/bolts, cover with vacuum line or caps, mount Quick Fists (mentioned later), and reattach crossbars.
MaxTrax and X-Bulls (and most other boards) will nest on top of each other, so you don't need really long bolts. I then just strap them down as shown. Another option is to use longer bolts that are taller than the stacked boards, and to run a metal strap over the bolts (with holes drilled for the strap to slide over the bolts) and use wingnuts to clamp them down (my only concern is the wingnuts might work themselves loose bouncing down the road).
For the shovel, a pair of Quick Fists work nicely. They mount with one 1/4 bolt each (on mine 1.25" long? I forget) - 6th picture. The Quick Fists cost about $10.00 a pair, and if you can't find them locally, they're on Amazon (don't get the mini ones - they'll work, but I'd get the next larger size as shown in the picture below).
MaxTrax are about $300/pair; X-Bulls are about $75/pair. I got two pair (four boards), so ~$600 + vs. ~$150 +/-. Just sayin'.
I also have a roof-rack on my TJ, and can mount the X-Bulls and shovel the same way.