Base too heavy for downstairs neighbors

Try a board with two screws through the front and one at the rear,set on the floor points down and then set your sub on top,this should help,also if you make 2or3 boards your bass should be tighter and not travel so much down to the flat below but will not go away,sorry but the only other thing you can do is buy ear plugs for your neighbours.
Hope this helps.
CABLE GUY.

If the sub is not too heavy you can buy wall mount speaker brackets and mount the sub up by/on the ceiling in that corner. Just think of it as a mirror image of how it looks on the floor. When mounted this way you will still be taking advantage of the rooms acoustics, just like before, except your (downstairs) neighbors should be happier. The only problem is if your home is poorly insulated it could still "leak" down their walls. So it would help to use an outside wall as one or two of the walls that make your corner. Another option would be to move the sub to the middle of the room thus negating the positive effects you get from the sub being in the corner. It should, however, be a little less boomy and shouldn't travel as far.
-Dynami