roof rack bolts

I finally got around to ordering keys for my roof rack, sometime this week I should finally gain access to the channels on the top of the crossbars. What I need to figure out now is what to use for bolts. Once I get the end caps off I'm sure I can find something that fits, but I don't know if there is a best size/type or special order bolt or what. If it helps, the main need is for a kayak racks, but I may use it for other things as well.

I assume you're talking about how to mount to the channel in the roof, correct?

It's less about the bolt and more about the surface area you can gain with washers or other pieces of metal to spread out the load and grip area.

I'd recommend bolts with the head in the channel, pointing up, with washers between the head and the channel. Take oversized washers and grind the edges flat to fit in the channel.

Part of the challenge is the the bolt will spin if just turning a nut from above. You'll have to figure out how to hold the bolt in place.
Square head bolts might work too.

Or someone might have much better suggestion.

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Tee slot bolts.