Unless you got parameters... I cannot help and stand behind it. I looked at a 18 Btl neo in 7ft3 net and I liked around 100in2 of port about 33" long to get it to around 30hz. That said, I'm not even sure what yours is gonna model up at for your power... I erred on the high side of port area as it looks like that driver can take quite a bit more power than you got... good luck
Edit....
is this it?
Fi btl d2 18" ufo
FI BTL D2 18"
2500 W. RMS, 1.4 Ohn Coils
DUAL 2
Fs: 33.6
HzRe: 1.4 Ohms
coilQms: 8.28
Qes:.32
Qts: .31
Mms: 344g
Sd: 1210cm
^2Vas: 133.8
Spl: 93.8dB 1W
1mBl: 25.1 N/A
AXmax: 28mm
Rms: 2500W
Sealed box: N/A
cuftPorted box:4.5-8.0cuftSub
OD: 18.500”Cut
ID: 16.750”
Mounting depth: 11.125”
Displacement: 0.18cuft
Looks pretty much the same to me modeling it up... it'll take a bigger amp for sure to get it near rated