A walking foot attachment is not the same as a walking foot machine. Unfortunately everyone like myself is looking for a walking foot machine and sellers seen to know that.
I use a Singer 66 (circa 1905) and a Singer 29K53 (circa 1934). The second one can easily go through 8 sheets of leather, or even through finger bones.
Failure, unfortunately, as I am still learning about seam allowances.
I also made it *much* more difficult for myself by using 8mm foam as the padding.
The idea was to bind all the external seams with tape, but two layers of foam was too thick to go under the "foot".
This made the case half an inch too big in each dimension for a tight fit.
@bigegg ,
Put up a pictures of your sewing machine showing it from the front and from above the needle end and one along the machine with the end plate removed the one that hides the up & down shaft etc,
There might be a solution to your difficulty
Using gazebo fabric , it's wise to make two pockets inside out then turn them so the outer ihas the seams inside . slip in the foam inserts in , they can be sewn if you feel like it and close trimmed on the seams use one long strip of foam and two small side panel to make the foam pocket , then insert the inner pocket so th seam are inside adjacent to the foam then all you need to do is run up the top edge as you put the flap on it .
If you work back wards make the inner pocket of material a pinned up close fit to the instrument , then add the foam sleeve pocket then add the pinned up tight outer pocket you ought to be able to get a good fit , Now turn them the right way round and run up the top edge
PS putting Velcro on the outer cover in the right place is useful as you can put the other Velcro on the flap , I also put a belt hoops on my outer covers .
I've had to carry an EpiPen and some other emergency items for several years. The nylon sheaths were a lifesaver, as I always had the medication and two EpiPen's with me that never got damaged.