Al West has full on windsurfing vans with racks, $299/wk for the mid age ones, $199/wk for older ones, plus insurance. Not the cheapest option but they are big vans and the racks are great if you have lots of gear.
Kimo’s Rent a Car has mini vans which are fine for 1-2 people plus gear at $140/wk or $500/month including liability insurance.
Others
If you’re staying longer than 6-8 weeks I’d consider renting for a week or so and trying to find a van to buy. It’s a bit of a hassle but can be much cheaper. Craigslist and the noticeboard at Kanaha are good places to look.