Hmm i'd say use some like wood to make a "Base" (A kind of skeleton to put something onto) Then take plastic or cardboard if you have to and shape it onto the skeleton.

Okay, there are several ways to approach making a blaster like the Deece-17M from scratch. Firstly I'd recommend making it from sintra as that's a very easy to use material and also very forgiving. Secondly you need templates. to do this you'll need to make a 1/1scale drawing of the DC-17M and then chop it up into manageable chunks, layers if you like that can be placed on top of each other to give you a 1 dimensional image of what you are going to make. Then mark out and cut out the sintra. You'll want 3mm sintra as you'll need to layer it all together and the strength comes from all the parts together. You know! It's not that easy to describe to someone who's not infront of you how to make templates and put it all together! lol

Okay, I'm gonna help you cheat, unless you specifically WANT to make a DC-17M in which case I'll try and take you through it stage by stage (this is how I make pistols and EE-3's so I KNOW it can be done it's just complex to explain) then I'll pm you an addy of where you can buy one.