Buy plain maltodextrin in bulk from Bulk Powders or similar. It’s extremely cheap, and you can flavour it with anything you like – I use orange squash. Tastes much much better than SiS.
A tenner for 5kg:
https://www.bulk.com/uk/maltodextrin.html
Just looked at Stealth Training – £8 for 500g, compared with £1.64 for 500g on Bulk. Seriously, Stealth?
Torq is maybe a little bit better if you are on a flat out race, perhaps because of the fructose, but you can buy fructose just as cheaply, and also electrolyte mix. But it’s even more expensive again. The only issue with mixing your own fructose/malto and flavouring with squash is that the results end up pretty sweet. Not a problem with plain malto though.