I agree that the PVC-coated rigid costs a bit more, but here's an 'outside the box' idea since you said that it's for such a worthy cause.

Since PVC-coated conduit is used in water, wastewater and chemical installations, ask around and see if there are any Contractors who have leftovers from a previous job that they're stuck with.
They might be persuaded to donate a few lengths...perhaps for a tax writeoff.
The local water/wastewater plants may even have something usable in their scrap pile from a renovation that you could have for the asking.

Based on the pictures you could do the whole job with several straight lengths and may only have to buy a few fittings and boxes.