I agree a cup is best. I find them quite bulky, though.

I've experimented with the foil trays. You get fewer leaks in the foil if you fold it less. If you merely flatten it, you may get something that will fit in the large plastic freezer bag the medkit was in and which can be reshaped without holes. If you try to make it fit a smaller PSK by folding it over once or twice then holes are probably inevitable.

In practice I found the leaks are often quite slow, little more than a pinhole that drips. I found that I can boil water in them despite the leak, provided it doesn't drip onto the fire and put it out. It can help to pinch off the hole to make it slower.

I've also found even a leaky tray is a good support for something else that may be water-tight but not fire-proof. For example, a plastic bag. Put the water in the bag, the bag in the tray, and the tray over the fire. Support the tray, not the bag. Any exposed bits of the bag are liable to melt so its probably best to cut them off.

In theory you can boil water in a plastic bag on its own, because the water takes the heat away so the bag can't get any hotter than 100c. In practice I find the parts of the bag not touching the water melt, and I've never figured out a way to support the bag over the fire without more melting/tearing/spilling. The combination of bag and foil seemed to work better.