I need to support a top heavy gumpaste structure (windmill) on a sculpted cake. The cake itself is about five inches high. The bottom of the gumpaste structure is a 2" hollow round tube. I tried to cover the bottom with gumpaste and hoping it will be dry by tonight. The gumpaste piece is top heavy. and I'm not sure what type of support I should use to make sure it stays in place. Can somebody please advise. I need to complete this tonight.

When I have something that is top heavy, I usually run a dowel through from the top of the piece down into the cake to the bottom of the board. If it's that heavy that it will sink into your cake, you will need to place something underneath it and dowel (or whatever support you use) like you would a cake tier to support the weight. I definitely would place to gumpaste topper on after I got the cake setup.

It's hard to tell without seeing the structure, but maybe you can use some edible glue to glue a nice long skewer to the inside of the tube and stick it all the way down into the cake. I did a tall fondant clocktower that way. Although it wasn't really top heavy, just tall.