good advice from jason as always
do you have rebar cutter like this on
http://www.screwfix.com/app/sfd/cat...t/pro.jsp?id=16095&slotName=null&homeRef=null
if you dont have allternative larger bearing you to reduce to about 7mm
you just make the rebate the width you can and make the inner frame
2mm bigger than the cobined reduction suspect your cutter will cut 12.7mm
so 24 inches - 1inch +2 or 3mm 588mm
if you dont want to or cant rebate you can add a bit of ply mdf wood
to the back to make a rebate
if your going to paint it go for mdf 12mm cut from solid
in other words cut a square then cut out another square 23.5 inches
add another bit of mdf about 2mm thicker than the mirror
on the back from the outside edge to 1mm short of where the mirror
edge will be so you now have a solid looking structure with a rebate
you can also add a bit to your pine to give you a rebate
so what you have is your frame mitred and glued [no dowels]
you then add the bits for the rebate but going the full hight of the frame
holding the the top bottom and sides together then put a bit top
and bottom to complete your rebate
also if you screw and glue a bit of 4mm mdf to the back[2ft by 2ft]
it will hold it all together
big all