Guitar Amp/Aux Cab

Newbie here, prepare for some fundemental questions. My first job out of HS was in a cabinet shop and while I've moved on to a different career I still have a "hobby shop" with table saw and other power tools, so I'm not a newbie where woodwork -and especially cabinet making is concerned. I recently began taking guitar lessons. I'm using Fender Super Champ 15 Watt tube amp driving a 10" Fender speaker. I'm planning to upgrade in steps and want to avoid making expensive intermediate mistakes. Following are planned stages, with some attendant questions-and likely there are some questions I should be asking but don't know enough to ask yet. Feel free to make suggestions or ridicule my plans-I can take it!
1.) Build a Scoop Design Aux speaker cab for high-quality 12" speaker, unplug the on-board 10" Fender speaker and use this combination for awhile.
1.a.) Scoop design to use rounded baffles of thin but solid spruce with braces as per insides of an acoustic guitar (speaker mount area obviously reinforced, but the plan is to get the cabinet to reverberate and contribute to the sound output.)
2.) Current Amp power output is 15 watts. Eventually I want to upgrade to a 40-60 Watt tube amp with a single 12" speaker on-board and use my already-built aux cabinet to boost coverage.
2.a) I've noticed aux speakers for sale come in different wattage ratings. Since I intend to eventually use a higher wattage amp driving my aux. cab., I'll probably buy a 75 or 100 watt speaker. How will this speaker perform when driven with my little 15 watt Fender amp?

This is a strange one, don't make a speaker cabinet out of soft wood. The only reason why pine can be used for Fender combos etc is that they are open back cabs with no real acoustic properties, and of course a guitar sound is all about effect. If you are talking about using the scoop design for your guitar sound, it won't sound very good and will have a very limited range. Also if you use a scoop with the normal combo, there will be a phasing issue. This won't be as noticeable as it would be in a PA system, but might still have a strange effect. Scoops have been used in the past by a few Musicians but only on Bass. Don't do it.