looks good, nice 3d puzzle your making there. Looks like a four piece mold with two loose pieces, one for the very bottom of where the transfer port joins the cylinder for the piston skirt, and one for the other end of the port. Its always preferable to have cores like this open from each end but in this instance I think that would result in some thin and fragile core box pieces. I would make a box with steeply tapered sides to hold all the individual parts. Done right you wouldn't need to use any screws or dowels to hold the parts together. turn the box over and rap it and all the parts come out then you can pull each part off the core.
Are those registration button molds you have sitting near by for the extra plastic?
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I spoke too soon, It didn't look like you had the draft for a three piece mold. Your getting good with the bondo.