With formal wear, you are meant to have a 2" leather belt, which only has two belt loops fit to the kilt.
The formal sporran can be confining, as many are made to only have a coin purse, as the vest and jacket have pockets galore.
A casual wear, often no belt or a simple 1" leather belt is worn; a larger sporran is also more common.
The make "piper's pouches" which are rectangular , in the neighborhood of 8x4x3
Which can be forgiving of the "stuff" modern life wants. Especially as a sporran can get crowded with a wallet and a cellphone stuffed in.
A belly-band holster is generally the simplest answer. If buried under a lot of layers typically.
The more typical issue with a IWB holster is that the vest needs tailoring to better be a cover garment.
If there is a local Scot's tailor about, getting a couple of patches of kid leather installed where the clips to your IWB grip best is probably a good investment.
I know some folk who use a small-of-the-back holster, since you often wear cover garments, and the SOB/MOB often means fewer tailoring needs.
I ran into this recently--last week--for Burns' Night. My DeSantis IWB did pretty well, it's all synthetic, so not as ugly to wool as a steel belt clip might be.
