I'll throw this one out there that i just drew up using bassbox pro. I don't sell these subs, I have installed some for customers though. Someone with a lot of experience with these exact subs could chime in with their experience. RE recommends 2 cubes tuned to 33hz on their site, with no mention of port area so I'll pull from a little previous experience with similar woofers.
This box would have subs up, port back and assumes 3/4" mdf all around.
Outside dimensions - 20H x 36W x 30L
Port - 18.5H x 5W x 23D This allows you to use the top and bottom of the enclosure as the top and bottom of the port, saving a few extra cuts and parts.
2 8' long 2x4's cut up into bracing inside the box.
This will give you just about 8.2 cubes at 33hz.