Urns like what someone's ashes are put in?
https://www.youtube.com/watch?v=4XkFx18wCPY or above type in Repurposed Shutter Planter Boxes Diane Wearing had posted info on them on this site
Oh, did you mean planters?
Cut to size, use L brackets inside. Attach 4 measure a piece of wood the size you need and attach to the 4 sides with L small brackets.weight. Add a 4 by 4 cut to the size you need to put inside and stand plant on, thus not needing to fill with anything but your plant.
Did you mean planters? If so be sure your doors are hardwood or are durable for outside /conservatory use. Then first draw the design you would like eg. just a square ?
Use the panels on the doors as a guide as to where to cut (If they have panels) otherwise measure 5 squares the same size (one for each side and one for the base). Then use 2" x 2" timber to make a frame allow for legs and upstands. Cut 4 pieces long enough to be the legs and then 8 pieces to fit in between that will take the panels. Fix together with Glue,Screws to form your Planter. Then paint or Preserve as required. Best wishes.