For longer-term development, there's vocal fry and false-cord vibration.
But probably the best thing you can do short-term is lots of rest. If you can manage to take a 2-hour nap before singing, it should make the low tones easier to achieve because the muscles are relaxed. The "two hour" part is to try to get one full 90 minute sleep cycle. The deep-sleep portion of the cycle is where the muscles are greatly affected. Any less than a full cycle may provide no benefit, or may even be counterproductive. Practicing may be counterproductive because it will increase the muscle tension.
And of course, breath-support. The deep tones need more support.
Edit:
A few more ideas. [I realize the localized event has passed. Hope it went well, @Ulf! So this is for posterity.]
In-line with the breath-support, one should try to rise up from beneath the note rather than reaching down toward it. This almost forces you to summon the note with enough breath behind it. Then the fully-relaxed larynx won't interfere with your deepest resonance. You get the air going then scoop up to tone. With really deep notes, you're never going to get much attack and you'll strain if you try. But if it's breathy, you can add volume and tone by adding more breath without any real need to tense-up.
If you can move just a little closer to the microphone, that should reduce your need for volume and make the breathy tone less inadequate (hopefully reducing your need to push it).