Hi Alex,
It sounds to me a bit more like you're still in the design stage, rather than having the picture of what you want and being ready to make it.
My understanding is that your characters are time-travellers? and you want the costume to show this by having different bits of the outfit being from different eras, from the past through to the future, yes?
OK, one way to approach this is to think of the character you wish to portray. I'm assuming this is his everyday clothing, and most people wear a particular piece of clothing because a] it's comfortable, or b] they think they look good in it. Oh, and in this case c] because it's what they are familiar with (eg. jeans for the 20th century).
Now taking jeans as the example - they're a good place to start your costume because they're easy to get (you've probably got some already, right

) and they have been worn (by men) for quite a number of decades. The down side of them is because they are something that a lot of people today wear anyway, some people may think you're not really wearing a costume, or only half a costume.

So it might help to think more about a particular character from the book....
- Which time and place was he born? (eg, 13thC Germany, 15thC China, 20thC American, 23rdC somewhere-not-earth?) . This will give you a basis for his clothing.
- What does he do in the book? You call them knights - do they fight? Is it organised? Do they have time to get changed into specific fighting gear, or is fighting part of their usual activity/lifestyle? If fighting is a usual activity, then I would think you'd need clothing that wasn't too loose, or would impede you in a fight.
- What is his personality - is he a suave and sophisticated James Bond type or more an Indiana Jones sort of guy? Is he serious or whimsical, studious or a bit of joker? These will all reflect in his clothes.
- Where/when does he travel is this book? This will also give you a clue as to the sort of clothing he might be wearing. Eg. if he rips his shirt fighting, he's going to get a replacement from whatever time/place he is at the time, or maybe he picked something up while he was there because it caught his eye (been there, got the t-shirt

)
Maybe if you can give us some more specific detail on the character you want to portray, we can make some suggestions on how to dress him
Hope that helps.