The final was left on a knife-edge after the Forman IT Eagles went down 52-38 in the first leg in Suffolk tonight.
The Sussex club fought a rearguard action after their hosts threatened to take a stranglehold on the tie in the early stages.
Eastbourne team boss Connor Dugard said: “It was tough going, which we expected, but the boys dug in and have given ourselves a chance of taking the cup for the fourth successive year.
It is going to be massive at Arlington on Saturday night and we will aim to hit them hard early on.
“This cup final could go right to the wire.”
The second leg of the final gets under way at Eastbourne’s Arlington Stadium at 7.30pm.
Fen Tigers: Drew Kemp 15+1 (6), Josh Bailey 12+1 (6), Ryan Kinsley 9 (5), Jordan Jenkins 7+1 (5), Sam Bebee 6+1 (4), Matt Marson 3 (4), Danny Ayres R/R.
Eastbourne: Tom Brennan 11 (5), Georgie Wood 9 (4), Mark Baseby 9 (5), Charlie Brooks 4 (5), Jason Edwards 3 (4), Ethan Spiller 2+2 (4), Charley Powell 0 (3).