Decomo,
The baffles sound best placed directly on the floor. Some folks tilt them up slightly, maybe an inch or two by placing a small wedge under the front of the baffle. I use rubber and cork vibration pads. I orient to baffles with just a slight toe-in with the phase plug pointed about 18" from my ears. YRMV.
The bass board is absolutely critical to speaker performance as well as keeping the baffle stable.
I suggest you check out the Decware Audio forum for additional guidance. Several DIY'ers have posted builds on their F-15 baffles, along with schematics with measurements. This should give you a leg up on your build and information about materials. Also, if you have limited woodworking skills, consider getting a friend to do it for you. One additional point, the F-15 drivers sound best mounted from the rear of the baffle with a rounded edge baffle opening in the front. This requires use of a router.
So definitely check out the Decware Forum. Members are always very welcoming and helpful answering inquiries. Good luck.
NB