I need the the desired output URL to handle be able to work with both a trailing slash or not.
Will these URLs be indexed? Since this is a Rewrite, not a Redirect, if you allow two-or-more name formats you will slam straight into Duplicate Content issues.
You started out saying "username". This sounds as if there could be a whole of them, so you can't write a rule that lists all the possibilities. Instead you'll need a rule-- or rather, a Condition-- that works from the other side. You want to exclude requests for the names of actual directories; I assume there aren't many that could be affected.
For the /username bit, decide whether you want it to display with or without trailing slash. Without is generally better-- unless you want your users to feel as if they've each got their own directory-- but you can make a rule either way. Or, if you really and truly don't care, put a /? at the end of the pattern, meaning simply "there may or may not be a slash here".
In the final Rule, make sure your pattern winds up with $ or you'll be rewriting whole directories by mistake :) Edit:
Oops, other slightly crucial question. Does the element /deleteme/ currently exist in any URLs anywhere? If so, you have to do the redirect-to-rewrite two-step to intercept anyone asking for the URL in its old form. Yes, you do this even if the URL is right there and working as well as ever. First
Redirect to the without-/deleteme/ version. Then
Rewrite right back to the with-/deleteme/ version.
Do a Forums search if you are getting dizzy. This is a popular question lately.