In 2015 I had to replace the two 18' weight-bearing joists under our deck - they'd rotted out to where the deck was dangerous.
From 16' the available length jumped to 20', so two 20' 2x10s it was, which I cut down to 18'.
I put together the roof rack extension mostly from spare lumber I had on hand. Notches are cut so it drops over the factory roof rack. I thought it was required because the factory rack crossbars are so close together. The extension was held in place with tie-down straps that pass through the van's door openings.
To provide tie-down locations at the front, I removed a couple of fender-attachment bolts under the hood and reinstalled them to hold nylon straps w/ grommetted eye holes. These accommodated ratching tie-down straps.
At the back, a hitch-mounted bike rack provided a good extension for the tie-down straps.
The load felt stable for the 8 km or so trip home, and I saved a delivery fee of about $50. Might have been $75, can't remember now.
Obligatory: Note the containers of used motor oil @ the RHS, ready to be taken for recycling.