Rick,
typically, you secure the conduits well using pegs and the plastic stand offs, and butt the bell end up against the plywood. Once the plywood has been stripped, you simply "tap" the concrete slurry with a hammer to remove it.
If you want, you can also form a window on the manhole (BTW, if you pour it in place, it's called by the ACI "Cast in Place" - sorry, American Concrete Institute). The problem with that method is the conduits must be grouted in afterward and that can be a pain, though it makes a very neat job.
Use the plastic forms rather than some other method, they make your life LOTS easier, AND they are designed to allow the 1 1/2" spacing to allow for the aggregate of the concrete to be vibrated in between the conduits, anything else causes a weak pour. Back to ACI on that one, that's why electrical specs typically call for the 1 1/2" spacing.
I prefer to cast them in place that way. Precast manholes typically come with a window you can knock out, but we're back to the grouting.
If the bottom of your manhole is below the conduit bank, there is little choice but to either 1) pour the manhole in 2 pours, [that allows you to backfill up the the level of the ductbank] or 2) cast a window into the structure, or 3) core bore the holes after the pour.
Geeez, did ANY of that help?
