Sherpa Roof Rack Bolts

Hello!

I’ve been trying to attach a Rhino Rack ski and fishing rod holder to my Sherpa roof rack. It comes with a universal clamp, but it makes the rack for my fishing rods sit pretty high. I was considering removing the universal clamp and bolting the holder directly to my rack. However, Sherpa racks typically use a special roll-in T-nut to attach items, like my water port.

My issue with this approach is that I’d need a pretty specific bolt length to make it work. It would be much easier if I could use a carriage bolt and bolt it from the top. I’ve seen many people mention that a 1/4"-20 bolt fits, but the head is way too big to slide into the track.

Does anyone know what size carriage bolt would fit into the Sherpa rack? I’m fine with removing the crossbar to slide them in, but I haven’t been able to find anything that works. Or is my only option the roll in t nuts...?