So, using the regular fingerings to slur from the high G-sharp (3rd ledger line above the staff in bass clef) to the E-flat five half steps below it doesn't work using the normal fingerings, at least the ones i use.

Anyone know of alternative fingerings?

This is tricky with most fingerings.

One thing is that you might be slow closing the half hole (LH1). The other thing is that you may be tightening the embouchure for the G# rather than using extra breath support. Practice playing the E flat first then slur up to the G# without altering the lips but using more air support. You should then find that going back down to the lower note will work. Plenty of reed in the mouth helps as well. This will probably need quite a bit of practice to get it reliably.

If the E-flat is a short note you might get away with playing it using just fingers 1 & 3 with left hand.

Good luck