Technically, the Peachy team has said that slightly tapered walls do not seem to affect printing small objects very much. I had considered making acrylic reservoirs, but the acrylic is hard to cut without shattering it, and it's expensive. I found baseball card holders to be too cheap to be replaced, so far. Everyone is interested in good reservoirs, though, so keep us updated. Do not use ABS, though, it reacts with the resin.