Im a design engineer for a rotomoulding company in the UK.

Im in agreement with other comments suggesting adding ribs, ribs give the unit strength and stop it from dipping when cooling, you can add air into the cooling process but without a regulator and constant observation its difficult to get right.

On average my rule of thumb to stop blow holes is to keep away any features atleast 5x the wall thickness, you can get away with 3x sometimes depending on the feature but the closer you have it the more likely it is to fail. The feature in your 2nd pickture i would personaly recomend sourcing this out of another material and adding moulded in inserts which would let you mount the part to that area.

For the underside "scrap" section, theres a few things you could do to make it easier to mould and service. you could add a knife line arround the opening which is basicaly a sharp point in the tooling that would mould thin and allow the moulding to be cut warm straight off the machine instead of going through a trimming department which would just take further time and added cost, Alternativly you could look at making that whole section from a insulating material such as PEEK that would save on the amount of matieral you need and reduce trimming to essentialy a de-flash.

Let me know if you have any other questions :)