I remember Barry Waterhouse and others saying the holes were in the wrong place, they should have been on the bottom not the top of the two 'arms' of the subframe, as they had been originally drawn that way, but were made incorrectly. If the subframe is removed from the car, then pour in lots of the waxoyl, warmed as suggested, stand a can a bucket of hot water as it flows more easily when warm, then move the subframe around to fill all the sections and turn over too to preserve the top as that can rust.
If the subframe it attached to the car this is somewhat more difficult! I think I put some tape over the upper holes to stop water getting in, I am not sure where would be best to put drain holes on the lower, if you fill the subframe with wax and water gets in can the water get out?
I did hear of using expanding foam in a subframe, I am not sure if that is a good idea, if water gets in and is then trapped by the foam and the steel is kept permanently wet, which then leads to you know what....