plastic is for kids toys not toilets. buy a couple of wax seals, you always return one. test with one seal tighten bolt slowly, this is a test fit , you be trying to rock the bowl. seems solid, know rocking at all, carefully tighten bolts. do not overtighten, you will compress the waxWhile you are at it you can also purchase metal kit - two half moon clips something like these that go around the plastic flange so the bolts will hold.
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You may also want to use two wax seals or some of the newer options.