Fortunately, using custom seeds does not disable achievements. If you have unlocked the customization rune, then you can run a custom game using seed #792769 (as of November 16, 2020). This seed has a rift in the opening level ("Prisoners' Quarters").
To unlock the ability to use custom seeds, you'll need to acquire the rune by killing an elite archer or zombie that is in a room in the "Ramparts," near the beginning of the level. Once you have the option for "Custom Game" on the menu, you can enter a specific seed number in the second customization screen. You can change other settings as well, but be sure not to enable any setting that disables achievements (these are all indicated with a lock next to them).
Unfortunately, as noted by Fshguy and JHunz in the comments, the seeds apparently change along with each update. The original seed #804327 no longer generates a rift in the opening area, and now the rift in seed #000119 appears to have moved, then disappeared in the latest update. I will try to update this solution whenever someone notifies me that this doesn't work. Latest seed will be at the top of this guide, along with a date stating when it was reportedly working.
The map screenshot is no longer terribly helpful, but this should show you what you're looking for if you've never seen one. Note the faint glow at my character's feet, as well as the text letting me know I can use the rune.