-
Corner clamps / frame clamps are available at a variety of outlets for a variety of prices - Lidl usually have them once a year at least. They will make your job much easier.
However, if you want picture framing glass you will have to buy that from a picture framer, and if you want anti reflective glass it will be pricey, so why not give them the job of framing it while you are there - plus you don't have to worry about getting a large, thin, expensive piece of glass home on it's own.
Alternatively, A0 is 841 x 1189 mm, which is only 200 mm longer than your map, so you could just buy an off the peg clip frame and throw it in, with a large bottom border.
I want to frame this
https://shop.ordnancesurvey.co.uk/mars-western-arabia-terra-limited-edition/
Due to its massive size it's hard to find relatively cheap options. I have a decent saw and a mitre - I'm not an idiot. Is it reasonable to assume I can make a frame myself?