Don't discount future horns yet. The best you can do at this point is to place the woofer as close to the top as possible for future flexibility.

For the t-nuts, drill a 1/4" hole right through the baffle for the bolt. Then from the back, drill a wider opening for the t-nut, but only as deep as it needs to be. Use a drill-stop to control depth. Masking tape on the drill bit works too. Then like you said, use a power driver and bolt to pull the t-nut in.

I've never heard of Acoustic Horn, but for the price they charge, I would consider a pair of Martinelli horns. Gorgeous and sounds great.

Let me know if you plan on flush mounting the woofer. I have a couple of lessons learned from first hand mistakes I want to share with you.

Gar.