Understood, thanks for the pics. Only things I can think of are: 1) The included filter is somehow the wrong one. On their website listing it cautions "
* CHAMPION FILTER ELEMENT: COF013 (old partnumber: X301-X356) WILL NOT FIT!!''. Maybe one got mixed in somehow? I have a spare or two of the correct filter and it measures nominally 35.5 mm thick metal endcap-to-endcap; 2) The aluminum insert that installs below the filter has been made incorrectly and is too tall? IIRC my insert was essentially flush with the opening (exclusive of the lip to seat on the rubber portion of the filter), or; 3) The cover has been machined incorrectly and the recess is too shallow?
For these last two, I might contact the folks at Heiden to source the correct dimensions that you can compare to, prior to spooling up the end mill. Per your pic, I don't remember that pattern in the floor of the housing but maybe it's just me. I also have the smooth cover without the fins, but I would think the inner details would be identical.
Best of luck, sorry I can't be of more help.