I have been wondering about the same problem - (I want to know if Neolithic astronomers 7,000+ years ago had the ability to magnify images of comets to give them a vague/reasonable idea of their actual shape)...

The answers here have given me a few ideas, such as grinding stones into bowl and/or dome shapes that are given thin layers of ice... A camera-obscura type device might then be used to focus any image onto a back wall. (Perhaps something as sophisticated as the Newgrange White House in Ireland)... This would entail the Bowl or dome would have to be carefully positioned to direct the available light into the correct orifice from which it would be cast into the darkened room.
Clearly this would not be possible for someone living alone & off-the-grid, as yourself... but I have had another idea that "might" work.

A bladder made from an animal intestine that has been filled with clear water and put under a high pressure to stretch the skin as thinly as possible "might" do the trick of enlarging an image. (It would probably be quite blurry, but I am sure it could be done). 

Much quicker than building Newgrange!

Cheers!