You might want to look at the smooth columns on the Fieldstone Accessory mold (just under 1/2" diameter). Unlike dowels, this way you get decorative capitols for the ends. There are fluted columns on the Roman Temple mold, which are closer to 3/4". You might also want to consider the 1" diameter smooth quarter-sections on the Station Builder mold (which work GREAT with the capitols on the Gothic Arena Accessory mold), and the few additional smooth blocks are a huge help with the small number of blocks on the Roman Temple mold.

Even USG Hydrocal (NOT Woodland Scenics Lightweight Hydrocal) is closer to 3X the strength of PoP; Hydrostone is closer to 4X-5X as strong for only a few cents more. If you let instant mashed potatoes sit long enough, they'll probably get HARDER than PoP, but I won't vouch for their ability to hold detail. So-called "dental plaster" is often only 20-50% stronger than PoP, for a lot more money. It's a step in the right direction, but only a tiny one, when there are much better materials out there for less.