Someone probably tried already but tried printing a simple box to a piece of plasterboard to avoid curling corners without the use of heated bed and raft/brim.
Recently bought a Anet A6 (rather impressed) with the aim to rebuild and increase build volume. Just seeing if large PLA prints can be done with hot bed so I don't have to build a large PCB hot bed.

Best results so far. No curling what so ever (to naked eye). The print sticks firm to the paper covering the plaster and the paper does not lift at all unlike masking/blue tape when hot (either by heated bed or heat of extruded PLA).

Downside is paper need washing off under warm tap for a few seconds. Small price to pay for nice flat print.

Anyone have any suggestions of similar bed materials that are rigid enough to resist curling of large prints (500x500?) but maybe flexible enough to bend bed material to peel from the finished print?