Just a warning there John, these mirrors might be just aluminised window glass. At 15 X you'll definately need optically polished glass to be flat enough not to distort your star images. I've allready been down that route
It doesn't need to be any particular wave rating or anything, just ground and polished any old how will smooth th eusual float glass enough for low power optical purposes.
I'm not sure of a an off the shelf source of such glass. perhaps search on Cloudy Nights Forum for sources.
This commercial unit from Oberwerk in the USA, uses optically polished mirrors and would be well worth the cost at $170 USD. Remember of course the view is reversed which has put me off pursuing it any further.
http://www.bigbinoculars.com/mirror.htm
A paralleogram mount makes the best sense I think if you want a natural view.