I believe that 4ft^3 enclosure from the DIYSoundGroup is designed for the HT-18, so if that's the case it should work fine. You could increase the perceived volume by adding 1lb of loose-fill stuffing per cubic foot, which probably isn't a bad idea anyway (reduces the backwave off the driver).Looking at snagging 2 HT-18's before they go away but have some questions.
From what I've seen these model good in larger boxes. What I'm looking at in WinISD as the best option is a 6 cubic foot box. I cant really go bigger than that with the space I have. Does anyone know of a flatpack for one of that size? I see the 4 cubic foot flatpacks at DIYSoundGroup and Parts Express but I dont see anything bigger. If 4 cubic feet is fine then I would just buy one of those as they are quite cheap for as high quality as they are.