I've never had a need to replace an unseen Thunderbolt large laser, but if I had to, here are a couple of ideas I might consider.
A: CRK-5003-1 Crockett Arm <20-778>
B: Rommel/Patton Heavy Tank Turret Barrel <20-750TRT> ($1.85)
C: AWS-8Q Awesome Arm <BT-270>
D: WHM-8D Warhammer Barrel <20-201F> ($1.85)
(Note, those with prices are available through the Salvage Yard. Those without, you would have to contact IWM about)
You can see from the picture that all three arms are about the same size, and it shows where I would make my cut and what details I would cut off (I'd cut the second fin off the Warhammer barrel, but from the angle of the picture there was no way to indicate that). The Rommel/Patton barrel would take some extra work, as I would also round off the edges by the cuts to make it fit the design better.
Overall, I like the Rommel/Patton barrel the best, because once removed from the turret it makes it harder to identify and has enough detail to be interesting... Second comes the Awesome barrel. This barrel has almost the exact shape and lines of the original, with only the details being different. Yes it is a little plain, but I might be tempted to see if I could carve some of the detail back into it... The Crockett arm has the best detailing of the bunch, but I've already used it in a conversion, so for me in comes in last. I don't like to reuse stuff if I can help it... Which by default make the Warhammer barrel an unenthusiastic third. There is nothing wrong with it as an option, there are just better ones to consider first.
The grafting is pretty straight forward. Cut the remains, if any, of the original barrel off. File and sand. Cut the replacement barrel where indicated. File and sand the end. Make your starter hole in the barrel dead-center. Drill. Glue in your pin. Center the barrel up against the stub of what remains of the original laser and press to mark the spot. Make your starter hole and drill. Glue in the two halves together. Fill in any gaps with green stuff (or whatever self-setting putty you prefer).
For the handle, I think I'd just use some .040 x .050 (or .040 x .060 if .050 is not an option) strip styrene. If you notice from the picture, the left side is a bit thicker than the right, you could get some .040 x .080 strip styrene, but that would add to the cost. I'd probably glue two pieces of the .040 x .050 together, and then carve it down after the glue sets.
The pod/barrel/whatever on the back, is a bit more difficult (an potentially expensive). I think I would use brass tubing, starting at 5/32. Because most of the tubing fits inside each other, I'd keep gluing smaller diameter pieces inside the other until I got down to the width of the wire I was going to graft it on with. When I got down to 1/16th, I'd make that piece a little longer, to make the stubby piece on the far left. I'd also try gluing in a piece from the left that was smaller than my graft wire, but not have it the whole length of the larger pod (if that makes sense). The reason is that I'd want my graft wire backstopped against something metal. Then I would taper and round the edges with green stuff... Of course the big problem with this idea is cost. At $4-$6 per package of tubing, and 5-6 layers of tubing, it adds up pretty fast.
Anyway, those are some ideas of how I might tackle this...
Caz