Machining is the best and most accurate solution. It's has already been discussed. Martin only need just over 1/32 of an inch removed from 2 tabs.
If Martin takes the lids off, he will save a about minute labor per box. He can expect the following prices:
100 boxes , 1/2 hour setup $50 (standard shop quote) 1 min run time each around $1.50. ~$200 total for 100 pieces. ~$2/ea.
500 boxes, the run time can be negotiated. $50 setup, ~$1.00 each, ~$550 total , ~$1.10 ea.
Crazy home version: Jury rig a wide flat tip high power soldering iron to a drill press. Set stop to minus 1cm.
Smoosh down one tab, slide sideways, handle up, smoosh second tab, slide sideways. Lots of plastic slag, but true to dimension.
Flick slag off is necessary.
Smelly melted plastic, Do this next to a door with good fan sucking outside.
Set up ~ 1-3 hours, runtime each ~30 sec ( minus cover removal) I know its nuts, but it would work. Martin saves or after the stop gradually lowers because of drill press vibration, he realizes he should have just gone down the the local machine shop. ..... The plot thickens....