I am not sure what you have in mind when you say a modified TL606, but I would like to hear more about it.
The internal volume of his cab is very close to a TL606. Maybe he could use it as a starting point for what you are suggesting. Maybe some added internal bracing and different porting combined with a different speaker.
If it were me, and money was tight, I would live with the cab until I could afford something I liked better.
I have a TL606 with an EV15L, (Mesa Boogie Road Ready Diesel), and I think it sounds pretty good with my old Ampeg tube amps. It would not be very difficult to build one of these with basic carpentry skills and tools. The hardest part would be to make the exterior look like a professionally built cabinet.