20mm thick solid lipping. Leave the stiles and top and bottom rails overhanging by 10mm all round to create a groove all round the outside. The lipping should then be tongued into this and glued with a D4 glue.
I know it sounds a bit flaky, and it certainly isn't the ideal way to go about doing an internal door but it does work. I know because I've tried it in my own workshop as an experiment several years ago.
I'm assuming you're asking because you don't have the means to make a traditional door, in which case it's an acceptable alternative. (just)