Need sub design suggestions

I am upgrading my system to surround and a new TV (yes.. the dinosaur must go). Currently I am running a pair of Cornscala's driven by a Krell KSA 150 but that will be replaced with a 5 channel and I would like to drive a pair of subs, positioned just inboard of the Cornscala's. I will use the Hafler 9303 for the subs for now. I would like to keep it under 18" high and 18" wide, depth can be up to 28". I have the plans for a Fitzmaurice Tuba 18 but I am open to other designs. Any suggestions on where I should look?
 

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That amount of space would give a good 3.5cf, BFM T18 was based on old school drivers, HT/Car subs have changed a fair bit since then and may not model like an old school driver.
Having the subs front and center may not be a good spot acoustically a small subs few spread around the room may work better.