Depending on what type of rack you have on your roof, you can use of the hitch bed extenders. These are normally seen being used in their short configuration for use with a pickup bed, but most types can also be used in a taller configuration for use with a rooftop. There are some pictures here, but I also recommend looking for one that has the extenders on the side that would prevent anything from sliding sideways.
https://www.amazon.com/Erickson-07605-Pick-Up-Extender-Junior/dp/B01CADYILQ/
I bought this one for my 12' kayak. I have a short bed f-150 with a tool box in the back and this thing works wonders. I'm in the hill country, so driving up and down hills and around bends is the norm. There's a bit of action/movement from the hitch, but if you tie the yak down with cambuckle straps to the T-bar, it doesn't move at all. It's very well built/welded/etc. You do have to buy an extra pin though, as it only comes with one and you need one for the hitch and the T-bar. Highly recommend this product.
too many times i've been annoyed at my lack of ability to transport a couple 4x8 sheets for a small project without renting a truck/trailer.
cheap pickup bed extender (shortened by 5")
Voilà!
I’ve moved a 20’ extension ladder with one of these in the vertical position
Erickson 07605 Pick-Up bed Extender (Big Bed Junior, 350 lb. rated) https://www.amazon.com/dp/B01CADYILQ/ref=cm_sw_r_cp_api_fabc_DBAXFb74N6JQD
And a blanket on the roof of my 19’ truck, but the pipe may not be rigid enough. If you have a pickup and delivery is >$50 I would but the rack. (Mine is from harbor freight)