Hi Simon,
As you know we usually use cheap tarps from ebay because of sparks from the firepit!
I have considered making a cover using canvas from an old frame tent or caravan awning, but if using a firepit with a chimney, you do tend to get the occasional spark hole so it would be a shame to put all that work in and end up with it holey!
Might be ok with an open firepit though
For just a top cover we have single use one-man protection type of things, available from the military suppliers at shows for around £3.50 - the material is better than the cheap tarps and is just the right size. Plus you get some very sharp pegs and loads of para cord as a bonus

That's what Patrick & I had on each of ours at t'Pit last weekend.
However, for best protection we use tarps big enough to cover the top and one side - that way you can protect against the prevailing wind, or use one long enough to go right over to create a tunnel, though the normal size tend to be too wide then, so require doubling up.
Let us know what you find to use - might be helpful for others
Cheers,
Robin.