If you want something that ends up like this:

It’s quick and easy to – if you have a belt sander, the curvature at the end lets you do this:

Then I use Plasti-dip to put a few coats on the hand grip for a little padding and more friction. I posted the entire process (link below). I’ve also done this with exactly the reciprocating saw blades you propose using – I’ve got some in the garage now. If this is a fun project for you, have at it. If you really just want a reciprocating-saw blade with a handle and want it to be as light as possible, PM me and I’ll pop one in the mail to you. Probably on Monday because I’m heading on a Father-Son, Math-Fishing-Snow-Camping trip this afternoon (taking the middle-school math team across a frozen lake to a wildlife refuge cabin for 3 days).
http://backpackinglight.com/forums/topic/58657/
Other ideas for a handle include:
Make the finger indents, and then wrap the handle area with 550-cord or, better yet, a little bit lighter line. It can double as emergency cordage. I’d finish the ends by capturing them in a loop under the wraps, just like when you “whip” the ends of a laid rope with small cordage:

Use a good two-part epoxy or gorilla glue to glue some wood on each side. I’d suggest bamboo – I keep a case of bamboo flooring around as material for a variety of projects. Weirdly, (until you think about it) sawing and sanding bamboo smells just like mowing the lawn). If you found just the right length nut and bolt, you could make it really secure by bolting through holes in the wood / blade / wood. Once the wood is attached, sand it down and sand out any finger indents you want.
You might look at old kitchen knives at a thrift store and see if any of them have handle wood or bolts that you could reuse onto your blade.