pictures are worth a thousand words. :) Luckily our pal Sachu has been buying up the last few YH/HP1's and this pair was pristine ( best condition I have seen yet )
#1 - spare bit of foam or anything that flattens which will damp the pivot joint.
#2 - fairly dense felt cut to about the same size as the opening, when pressed down it should overlap the sides and damp the vents
#3 - creatology foam cut to fit the inner aspect , press fit into the cup
#4 - gasket "stuff" blu tak, bitumen etc , whatever you want to use to seal the driver to the front baffle, roll it into a long snake and press fit the driver into position.
Make sure the driver is aligned so that the tabs do not catch on the lower screws and so that the strain relief is aligned correctly.
Finally close it up - I start with the screws at the wire entry and then go to the opposite side - there will be a small gap
the gap should be no more than this - it is a firm press fit to close, and it will close smoothly and completely.
This mod will not be for everyone but it is the best performance I have ever had out of a YH1 / HP1 - as good as a well modded YH100 ( which is a bear to mod correctly )
hope that is clearer now than before.