How about 'not DIY' but an 'AIY' solution like the GSG Marty 15" or SQL Marty 15" kits?
Since the Monolith was mentioned above.. I'll use the Mono15THX ($1499+) for comparison...
CEA2010 (from Audioholics bench)
2 Ports open, Hz/dB
12.5/98.6, 16/105, 20/109, 25/113, 31.5/115, 40/116, 50/116 Stays 116 to 125Hz
3 Ports open, Hz/dB
12.5/90, 16/103, 20/109, 25/113, 31.5/116, 40/117, 50/117 Stays 116 to 125Hz
Marty SQL 15", Tuned at 21hz
12.5/99, 16/107, 20/114, 25/116, 31.5/116, 40/117, 50/117 Stays 116 to 125Hz
Cost.. $576 for two kits, $622 for two drivers, $580 for 2x1200w DSP amplification, $320 shipping & $100 for glue, brush & 1G of Duratex ($2198pr) *plus a weekend of fun!