You can use plasma treatment on both surfaces of glass and PDMS slides or box.

Dear Ling Wei I agree Xuhua Wang about plasma treatment, but it could be quite permanent and you will need cut the PDMS box from the glass surface at the end of the experiment. May be this method could do the job if you use a razor blade. Damage will be minimum to the glass surface and a very thin slice of silicone will be lost.

If this process is not functional for your experiment (I don´t know, but imagine you are growing cells on the glass surface encased by the silicone box... or depositing something from a liquid solution inside the box...) it could damage the cells or the deposited thing. Anyway the method could be applied, just cutting the silicone walls, without getting into the area inside the box.

If you are planning to hold a liquid inside the box, it should be tightly attached to the glass surface to avoid leakages. Pressure can solve your problems if the box rim is quite flat, so that it touches the glass surface in all its points. Of course, silicone elastic properties can allows conformal contact when enough pressure is applied, but the box walls must cope with such a "big" pressure without collapsing or buckling.

May be you have seen these experiment that children do: a glass of water full to the rim, a papper covering it and placing the glass upside down, so the paper seals the glass and keep the water (actually the pressure do that). The same thing could work for your experiment.

A similar idea: if you can attach a connection to your silicone box (or glass) so that you can make a vacuum, it will keep both things attached till the vacuum is broken.

Hope it helps!

Thank you both for the suggestions detailed recommendation. However is another process that's a bit more easily removable at the end - without the need for cutting?

Does Thermal bonding work? I've never tried it and am not very familiar with these processes.