You could just make a kilt liner, or several, so you could have - maybe one of Goretex, and a couple of a polycotton, so you could change it easily.

All you need to do is take a length of fabric slightly shorter than the kilt and put in shaping at the back using darts, have the smooth side inside - so the hems are all turned outwards, and to fasten it put two short lengths of strong elastic, or shock cord at the edges of the waist and sew on two buttons. They will not be uncomfortable if they are above the level of the belt.

To make a permanent lining you would probably sew a piece of fabric to the inside of the under apron and its pleat - something lightweight.

You could use the same material to line the fell, even if you did not cut it out and reinforce it, down to whatever length you like, either long enough to sit on or just enough to protect the rump when standing/walking.

Hmm - somewhere I have two ancient and threadbare kahki blankets I was thinking of making into a coat at one time - when the 'in' colours were kahki and cream.