I admire your desire to do the best that you can, and understand that you don't want to be opening up the box and adjusting the damping material repeatedly.
Regarding bracing, what Will33 said.
Regarding damping, imo there's a sweet spot for a particular woofer/cabinet combination. The right amount for my woofer and cab may not be the right amount for your woofer and cab even if we use the same woofer due to differences in our cabs' internal dimensions and tone goals. Fortuately, it isn't critical to bullseye that sweet spot.
Here's what I suggest: Making a "middle of the road" attempt on your first try, and plan on making one revision based on how that first try sounds. Given what my experience has been with the 3015 (non-LF), I'll offer suggestions.
Usually the most difficult internal standing wave will be in the longest internal dimension, and the easiest to deal with will be in the shortest internal dimension. This is because short wavelengths are easier to absorb than long ones.
The internal path lengths in a cab are short enough that the reflected energy is going to have many, many encounters with even a small patch of damping material within a fairly short time interval. It's not like a bare patch = that midrange energy survives unscathed; rather, it only survives the first bounce unscathed.
So as your ballpark starting point, here's what I suggest, assuming 1" thick convoluted foam: 40% coverage of the total combined surface area of the top and bottom of the cab (assuming that's the longest dimension), divided up however you want. You can cover 40% of each, or 80% of one or the other, or something in between.
Next, 20% coverage of the total side surface areas, again divided up as you see fit.
Finally, 10% coverage of the total front & back surface areas (consider the woofer cut-out to be flat). Might as well put it all on the back panel, so that would be 20% coverage of the back panel.
Now, re-assemble and listen critically to your bass, but you may also want to try fullrange program material, as for me at least that's easier to use for judging the midrange. I like to use male vocal + acoustic guitar. With the 3015 you should hear a slightly barky tone in male vocals, but not to the point where it's painful. And your bass should have pretty good thump in the low end, and a bit of upper-mid growl.
If the cab sounds lifeless and/or the thump isn't there, decrease the amount of damping material. By how much? That's a judgment call of course, but whatever you do will probably be an improvement. Maybe cut the total in half if you want to make a pretty big change. If it sounds good to you, it's okay to go all the way down to zero damping material. In some cases that works quite well.
If the cab sounds too edgy and irritating, then add damping material. Maybe double up if you want to make a pretty big change.
Hopefully you'll only have to go through one cycle of refining like this. I don't know for sure that this is the most efficient approach, but it's pretty much what I do.
If you're using threaded inserts of some kind, you can do this a lot. If your screws are just going into plywood, don't super-snug them for this testing phase. If you want to rehabilitate the screw holes at some point, get a toothpick and swirl wood glue around the inside of the screw holes. Let it dry for a couple of hours, and do it again. Then let it dry overnight, because you don't want the glue sticking to the screw itself. Now your screw holes should be good as new, maybe even better.
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