hidalgo,
I second the suggestion of Ramshot Big Game, as it's a spherical powder only slightly faster in burn-rate than H4350. The small granules take up less room in the case, and in my tests it's been just about as temp-resistant as H4350 at temperatures from zero to 100 Fahrenheit. (Apparently many of the posters somehow missed your requirement of temp-resistance, since several of the powders suggested are pretty sensitive, especially to cold.)
IMR4451 is a great powder, one I've now used to replace H4350 in half a dozen cartridges. So far it's provided essentially identical results with very similar powder charges, and like H4350 is also "short cut," so meters well, and also contains a decoppering agent. But because it's so similar to H4350 I doubt it would get the velocity desired with the 225 TTSX, because of the same space limitations.
