Hello Turpin

For the separate liner I just add a few inches of elastic at the edges of the waistline, then put a button hole in it, or attach a half inch hook and bar, and then fold over the edge with the sewing machine.

That seems to give just enough variation in the waist to make it fit snuggly from morning till night. If it gets a bit old it is easy to remove and replace. It is the same shape as the kilt but without the pleats

I use a fairly light material for the liners so having the double front is good, and it gives more freedom of movement. I make it at least long enough to sit on as that also stops the kilt getting too friendly.....

The buttons or hooks can be undone and the liner pulled from under the kilt, and it is not too difficult to replace it with one maybe of a different material, or even to take off the kilt in public and not cause alarm.