These are a bit more pricey but they should work for you. Bottom internal dimensions measure 21.125"L x 14"W.
http://www.greenhousemegastore.com/p...-trays-inserts
The bottom dimension of those plug flats certainly look smaller than a 1020, but if they say they do not fit, you have to go by that unless you can find a flat and try it out yourself before you buy.