I have done this twice.
Both times I've used the starting setup of a mini-campaign as specified in the TS or BG rulebooks, i.e.:
Bespin Gambit Rulebook wrote:
MINI CAMPAIGN SETUP
Before starting a new mini campaign, perform only steps 1-3 of “Campaign Setup” as outlined on page 7 of the Rules Reference Guide in the core game. Then, perform the following steps:
1. Build the Agenda Deck: The Imperial player builds his deck of Agenda cards following the guidelines in “Building the Agenda Deck” on page 4 of the Rules Reference Guide in the core game. Then, he returns all cards in his Agenda deck that are played as side or forced missions to the game box.
(Note: Obviously, this step is skipped for a one-off mission. --AB)
2. Upgrade Heroes: Each hero receives 3 XP, and heroes collectively receive 400 credits per hero. Then, heroes resolve a Rebel Upgrade Stage. When purchasing Item cards during this Rebel Upgrade Stage, Rebel players draw cards from the Tier 1 Item deck and draw ten cards instead of six.
3. Upgrade Imperial Player: The Imperial player receives 3 XP. Then, he performs an Imperial Upgrade Stage, but skips purchasing Agenda cards.
4. Set Up Introductory Mission: Players are then ready to set up and play the Introductory mission for the mini campaign. For The Bespin Gambit, the Introductory mission is “Reclamation” (see page 8).
(Note: Here, you would set up your chosen mission instead. --AB)
In my case, I went with Bunker Buster from Star Wars: Imperial Assault – Stormtroopers Villain Pack and Into the Wastes from Star Wars: Imperial Assault – Bantha Rider Villain Pack, since odds are I'd never get to play them otherwise. But any side mission would be fine. The threat level would be 3 in all cases.