Hello, I've also just bought an Alphard and have been doing the same research
It looks like these are a couple of options if you can find them or afford them:
https://www.prorack.co.nz/cars/toyota/alphard/2007/5doormpv-144414/117549
https://www.goodeleisure.com/treefrog-suction-mounted-roof-rack-cross-bars-7282-p.asp
I have a Canadian canoe so will likely go for the suction roof bars