McBonfyuh

When curing the mix, the wood will float so I set the pan between two bricks so sides wont bulge and  set  a 3rd brick across the pan pushing the wood down so it will be flush with the top of pan. Don't leave it to dry fully! When its still slightly grey it will save you a headachee   if run a putty knife along the walls of the block and pan as well. i do this three times as its curing.   now i have what looks like a block to fill the lower half of a pan the same as before but with no gaps between any of pans walls, this is for the lid. pour some graphite powder  evenly coat the wood before u pour same refractory with more plaster and less sand plot your holes and do some pilots while its still muddy then again right before almost dry. add a thin layer of plaster around edges and plop the1st form upside down on the the second.  its tedious but worth it...  before  joining u can slice out the entry  hole and for more consistant flame i cut a square of stainless and float it centered in the first form to break the straight flow from  purge out to uneven

will post pictures of one i have almost done with what stuff

Kind of late for my input but maybe someone else will come across this and find it of some use.

_yes the holes are quite large imo. alot of folks will cast ribbon burners and use low density styrofoam with birthday candles stuck in it and melt  the foam and wax later.  Don't waste time with this method the candles are almost perfect size though.

I have made many ribbons burners using a plasterers| loaf pan and filling it with a mix of silica sand clay and easy sand 20/45/90  minute plaster depending on comfort level.   I take  a block of wood copying the pans shape on a smaller scale  so the if tjhe mud filled pan would be 3/4'' from edge of the block on 4 sides and  the belly of of the pan.  pour refractory in. you want it about pancake batter thick maybelil thinner. this is a good gravity for overall strength/ heat refraction