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I think that it depends on the printer you have. On mine, it fits together pretty nice and snugly using just pressure. Two options for you:
1) If you want to print again, you can open up the OpenSCAD file and adjust the tab_spacing variable. I think you need to decrease it to make it a tighter fit.
2) Since you printed in ABS, you can make a really great glue out of 'ABS slurry' - that is, take some nail polish remove, and soak a bunch of waste ABS into it until it dissolves in a cloudy liquid. You can then use this to glue your parts together (effectively, it is welding the parts together). ABS slurry howto:
2a) If you want to glue PLA, I recommend this stuff: