I believe Erica got a 3" one to work, but wasn't happy with the results.
Batchbox rockets can be shorter than originally thought and still operate well.
The riser can be roughly 3 timed system diameter with no real issues.
That makes for an 8" batch box with a 24" tall riser, as measured from the base of the box, so fitting it under an oven should be a non-issue.
Flip and John Anderson have done some of the earliest work on rocket ovens and even kilns, and their rockets are all basic cob cooking rocket.
Here is a video of their work: