I have to do a treasure chest with the open lid. I'm seeking any advice on how to prop up the lid and how it should be made. I see many great treasure chest cakes on this website, but not much insight on how it was put together. Thanks.

You could make the lid out of an iced cake board OR gumpaste.
Place a dowel on either side of the cake, near the back of it to hold the lid up. Hide these w/jewels etc. Don't make it open too much - the dowels should stick up only about 1" to 2" - no more.
When ready to place the lid, pipe a line of icing along the back edge & tuck the lid into it so it is held in place as it rests on the dowels

I did a treasure chest cake with open lid. I got a foam core board and scored I with w knife about 1 inch strips this allowed for the lid to bend then I attached 2 dowels to the foam core i took a styrofoam circle cut in half (the width of the cake) bent the core board around the circle, covered in fondant. The bends in the lid made perfect "boards". I place jewels and ring suckers all over and put a longer dowel in the center of cake and put the lid on he put a peice of fondant to hid the seam "hinges".

Thanks for the replies and the tips. I'm trying to gather as much help as I can. I have to make this cake in about 3 weeks.