I've planed plenty of slabs on my SB and I've found that very little hold down support is required. As long as you don't try and take off 1/4" at a time, all you need do is keep the slab from sliding. What I do is load the slab with the bow up, then shim the rock out of the slab on both sides with business cards (a good use of those old fashioned things). I typically use some boards to lock it into place but generally, the bigger the slab, the less hold down required. Once I flip it over to the flat side, the slab needs almost nothing to hold it in place.
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2013 PRS 96x60x12 (Centroid upgrade)
4hp Spindle
12" indexer
Aspire
Rhino
Fusion 360
Ferrari 360
Prusa MK3S+
Prusa XL multi-tool