I'd have to disagree ,ever so slightly with David, in saying that moving the buckles/straps isn't such a difficult thing, of course it needs to be done carefully, but it will help you to find a few extra inches without too much difficulty. Of course it's not ideal from a perfectly fitted kilt idea, but it's certainly workable.
Extension straps are workable as well and don't need any sewing, and often the straps are hidden by a belt/waistcoat/jumper/jacket.
None of these solutions are perfect but they do allow you to keep wearing a favourite kilt whilst saving for a new one!