Those nipples just screw into a standard bung housing - 16mm coarse thread by their website. You will need to create enough space between the two housings though to give you the depth required. I don't ever recall seeing a bung housing that was long enough to have one bung screwed in from the inside and one from the outside (the nipple) . You could either weld in a box or use a well sealed heavy grade plastic if it's above water line and you don't want to do any welding as one option. Another may be mounting the housings back to front but you would need to make sure their is enough thread depth and that doing so didn't compromise the sealing quality of the fittings.