If the top cover is in good condition (it should be) remove the torx screws and remove the bottom of the ecu box there should be a seal around it in good condition if not you need to buy the whole lower part of the ecu box because the foam seal is not sold separately. You can seal it with Hylomar Universal Blue it is a non-setting sealant as extra safety measure.
Once the contacts start to corrode you need to make sure they are spotless otherwise it will come back sooner then you think.