How about using that hot melt glue that comes in sheets?

I generally use MDF with the 2-tape and superglue approach mentioned by Wal.  A nicer material that I have some offcuts of is “”Valchromat” which is denser, has a smaller grain, and flatter.  If you are careful with DOC an alternative is to glue down on a single layer of 3M blue masking tape on metal, so the DOC just gets through the metal.  Then boil to release the part.

Indeed – Valchromat is lovely, but I haven’t been fortunate enough to have any larger offcuts come my way.

Metal to metal before boiling off works great when you need a stronger bond, ie. for milling fiddly shapes where a lot of the glued area gets removed.  Just don’t glue over any screws that you may need to get to in order to remove the sacrificial bit of metal – that ol’ chestnut I know so well…  😂

OR direct (no tape) to hardwood worktop off-cuts like in the image below – once done you can dab on acetone which will degrade the super-glue and let you release the work without needing to use too much force.

When using my Shapeoko router, I found that for cases such as these and where it wasn’t either possible or desirable to use clamps, 3M Super Sticky clear tape worked well.  So well, in fact, that it was often difficult to remove the workpiece from the sacrificial board.  Not impossible, but it needs to be used sparingly.  One source is on eBay, here.

Not the seller I used, but has a wide selection of widths, lengths  and colours.

John

For many jobs I use the dividers from toilet cubicles (Washed first) It is about 1/2″ thick and very solid and does not bend easily. That is if you can find somewhere they are refurbishing some. Mine came from a school; that was having work done.

It machine well as I have just made a part from it.

The cut edge was done with a circular saw