As GW says...recut flatter, by local sailmaker, cash, in quiet times?
I fitted a clam cleat to my mizzen downhaul, which was then usable as a cunningham. Beware if your mizzen mast has no internal plug fitted; mine has.
Presumably, your outhaul is adjustable while sailing? It needs to be.
Also, if free/leeward sheet is hardened in too, once windward sheeted in, It acts as a downhaul/flattener. This assuming that boom is rigged "sprit style", part way up the luff. You'll need better blocks than the standard seasure crap...IMHO.
At least one boat (not me) has experimented with short strops on the aft most mizzen sheet blocks, so as to improve sheeting angles.
Have a look at Salcombe Yawl sites, they are VERY competitive (money no object) racing dinghies. Beautiful things...
Some seem to have battened mizzen, stays on mizzen masts...