The key is the concentration of Russian income in the western tts most likely to be captured by Germany.
So:
increase the overall Soviet income to something more like historical levels, but with the bulk of its industrial base (income) in the west (i.e. Ukraine, Belarus, Leningrad, Caucasus)
give the USSR a factory in the Urals, or allow them to transport one factory from the western tts during the game
This means that
Germany can still grab a big slice of Russian income with successful early attacks
Russia will still have a large area to draw income from even if the industrial west is lost
To balance, the number of tts in Russia might need to be increased, with tts roughly the same size as those in West/Central Europe. This gives Russia time to build up a large army, but not if it allows Germany to drive too far east.
This is of course also dependent on my own insistence upon:
1. No building units in captured factories
2. Unlimited (rail) non combat movement within controlled tts
3. Limited infantry placement in non factory tts