Do any of the answers for this question help? Karel Brinda has mentioned a few read simulators in the answer to that question, and has a thesis with more information.
Excluding INDEL errors doesn't sound like a good idea; length can still be preserved even if doing that, it just needs an adjustment at the end of the sequence. Note that if you're trying to model nanopore reads, what you're really modelling is the base-caller, rather than the sequencer. I mention this in more detail in my answer.
In most cases where errors are modelled, I find it better to use publicly-available data instead. Especially for nanopore data, there are unmodelled systematic errors in the base-callers and sequencer that can't be simulated using any programs (because they are unmodelled). The following paper would be a good place to start for cDNA sequences, which looks at single-cell data from mouse (C57Bl/6) B1a cells:
Illumina and ONT reads for that study can be found in SRA under accession number SRP082530.
I don't know of any recent D. melanogaster studies that have been done using nanopore. There's always the option of spending $1000 on a purchase of a MinION with an RNA starter kit to do the study yourself. Here's an older targeted gene study, but bear in mind that it was using an R7.3 flow cell, so errors rates will be much higher than what is currently available: