I want to invoke an arc angel. I have the universal circle. I have a space for the work. What is lacking? Do I need a crystal ball to invoke the angel or can I just invoke it?
No, you had enough when you were born to interact with spirits and command angels, and now you also have kickass tools to help.
Those are good, use them - and respect yourself as the Bornless, the One who Is, etc.
For invocation you can use only your voice or sigil opening…the second one is even better because you use the strongest link between you and the entity.(the sigil)
But where will the evoked spirit appear? If I don’t get to have the crystal ball?
I draw a distinction between INvocation, where you call the spirit into yourself and/or to interact with you, and EVOcation, where you are asking the spirit to manifest externally to yourself - there’s a huge amount of different opinions about these words, are they different, are they the same, so I’m just sharing how I use them. 
So you can invoke any spirit without any tools needed, though things like natural ability and practice will affect the perceived strength of the spirit’s presence.
You can evoke a spirit to visible and apparently external manifestation using a scrying tool, such as a crystal ball, bowl of water, black mirror, slice of agate (my preference), fire, incense smoke, mist and steam, or even just a patch on a wall, the sky, or anything else.
Dee as you’re no doubt aware used a crystal ball - it’s actually tiny by the way, I went to see it recently in the British Museum and it’s more like a golfball, I was expecting something more like a grapefruit-size.
But anyway any of these objects can be used and the simplest is probably a bowl of water, I want to experiment with using coffee (demons seem to like coffee) and you can use a dark bowl, or water with black ink in it, or a photo frame with the back of the glass painted black to provide a black mirror.
Once you have the technique, you can use any wall (my preference now we have our bedroom/Temple etc all in the same room) or even the inside of your eyelids, etc.