I've done popups by creating a form with the message, and using the
TIMER INTERVAL event, coding if for 5000 (millisecs) and then issuing
the CloseForm (macro or vba) on the ONTIMER event.

You'll need to create your own "MsgBox". To do this, create a form and set
it up to look like a message box. You would have the report open this form
using the acDialog window mode argument in the DoCmd.OpenForm call. This
will cause the code to pause until the popup form is closed or hidden, just
as the code pauses when you open a MsgBox. Since you are using a form, you
would then use the Timer Event of the form to have it close itself after the
desired time.