Sorry for the delay in replying! I don't get this far down the thread-list (usually run out of time after the Politics thread), and I thought we were still at your 'carrot' reply.
'Hollow Pumpkin' involves some possibly toxic objects (you know all the warnings on the Fimo package - don't use your cooking oven, vent, etc.?) Well, the hollow-pumpkin method I was taught involves a small Styrofoam ball (we used the crisp, sharp, crystal-like kind, not the packing-pellet opaque, soft-looking kind). Popular in craft stores for decorating around holidays.
Get a Styrofoam ball slightly smaller than the pumpkin you want to make, then flatten and squish it into a rough pumpkin shape (don't need to worry about grooves, this in only the inside shape).
Now, cover it with a layer of yellow-orange pumpkin-interior colour Fimo. While that layer is still warm from handling, cover the yellow-orange layer with a thin layer of pure orange. Push the layers firmly together, without distorting the pumpkin shape. Mark in grooves, trying not to make any holes all the way through the Fimo.
Do you want a stem? In which case, make a small green/brown stem-shape and attach it to the top of the pumpkin (for your first attempt, I'd skip the stem.)
Now, put a small hole someplace inconspicuous on your pumpkin, depending on what you're planning to do with it: either the blossom-end on the bottom, or where the 'stem' would be. (Middle of the back would be another possibility, but for your first few attempts, you may want a choice of what side to carve.) The purpose of this hole is to vent air from the shrinking Styrofoam, so make sure it goes through all the Fimo, and reaches the Styrofoam.
Now, heat the pumpkin according to the Fimo instructions, remembering that Styrofoam vapours are not to be breathed in. Once the time is up, haul out the pumpkin and let it cool.
Once it's cool enough to handle, and has gone hard, inspect your pumpkin to see if there's a preferred side to put the face on. For your first face, go simple: easy triangular features, well-spaced (so that the pressure of cutting out features doesn't crack the adjoining feature-edges.) Be gentle, you're dealing with a somewhat fragile, hollow object.
Cut the 'top/roof/hat' off last. Cut in at an angle, so that there's a bit of a ledge to stand the top on. (Like a real pumpkin) Once you have the top off, you can knock out the shrunken remains of the styrofoam It may be hung up on the bottom, a flick with some sort of implement should release it.
Then you should have the finished product: a hollow pumpkin with a yellow-orange crinkled lining, and a jack-o'-lantern face. You could skip the face and have a pumpkin-shell tureen for soup to go with your bread.
If you like mini-electrics, you could make a hole big enough for two wires, and put a small light bulb inside. Or, if this is too much fuss, just draw on features with a fine-tip felt pen.