I use cheap "clip" frames from IKEA as glass donors - much cheaper than having them cut to size. Fortunately they still come in "standard" sizes - 18x24, 8x10", 24x30, 30x40.

I've got inserts for 6.5x9, 9x12, 13x18 and 18x24 as well as the 5x7". Anything larger than 8x10" I'd much rather stick to a glass plate, at least there's a better chance of it staying flat that way!

The jam trick was an emergency solution when I discovered I'd forgotten to bring the sticky spray. But it does have the advantage of being water soluble as well as "environmentally sound".