I've carried 2 kayaks on my fiat grande Punto. I would suggest getting 2 pairs of J bars from eBay or elsewhere, makes it easier and some heavy duty straps.
They'll easily take the weight of a couple of kayaks.
Might be worth checking local Lidl stores for straps. They did have ratchet straps with 2 bungees for a fiver each. They're reasonably good quality and are strong enough.