have just decided to buy a womens underskirt and cut it to length
That wouldn't work, as it would tend to curl up once you cut away the bottom hem. Women's underskirts are much thinner than the purpose built "satin" kilt liners on e-bay which are actually made from polyester. The cutaway cotton T-shirt stitched onto an old jockey waistband could be rather warm in summer and wouldn't help the kilt to sit smoothly in the way that a nylon or polyester liner would. Personally I prefer a polyester liner in summer and a cotton one in winter, but if I am wearing a non-wool kilt, ie one which can be thrown into the washing machine I wouldn't usually bother wearing any liner with it as they do tend to restrict the feeling of freedom which is one of the great joys of kilt wearing.