Curiosity, how are you going to fit an 18" sub into an 18" box? Also, how are you going to power it?
The 15" HF series is best in a 4.6 - 5.2 cubic foot enclosure. Im using the 15" HF with a bash 500 watt amp in a 21" cube and it is simply awesome. You might want to considered the HO 15". You can put those in even smaller enclosures. They don't go as low, but the have a little less distortion. Its a trade off but might be worth it.
The issue is cost. You'll be looking around $450 - 475 when its all said and done with materials.
That is if you wanna diy 🙂
Cut out size is 17" overall diameter is 18" it's tight, but I've done it before. Down firing would work, you would just see the edge of the frame. Double baffling it would let the frame taper back a bit before it is actually in the box.
I modeled it in bass box pro and for a critically dampened box (qtc of. 707) it required 3cuft of total volume including driver displacement.
The 250 plate amp is on sale for $119 right now bringing sub and amp to $245 shipped. You could buy a sheet of mdf and a sheet of black laminate for about $70.
I would think you could do it for $350. But raw unfinished for under $300.