I used the 3/4" block inserts to make a few flats of blocks to hold the lettuce we started in the 3/4" soil blocks. They fit, but I'm not sure it us worth the effort to use the 3/4" blocks. Nearly everything germinates and grows, why not just make a 2" block to begin with. In any event, the lettuce starts look hood and I expect them to flourish in the greenhouse. Unlike the brassica, lettuce seem very tolerant of the heat swings the greenhouse presents.