If so what type of epoxy would be a good choice?

Are you trying to make a hunting mag?  I built a block from wood for my 308 mag to make a 5 rd, worked well but took 3-4 hrs of carving.  never tried epoxy.

Ned more info on what you are trying to do.... I have made blocks using many methods my favorite is nylon square rod...

Trying to get a magazine that was made for .30 mauser to feed 9mm reliably.  

.30 mauser is 1.36 inches long and 9mm is 1.169 inches.  Doesn't seem like much but the 9mm rounds are moving around a lot in the magazine and it seems to be causing a lot of feed problems.

For that if you cannot have an aluminum one machined....I would suggest JB weld brand "water weld" its (or its generic equivalent) play dough like form males it easy to shape and mold in to what you need and can be filed on for a better fit....

How about using ABS plastic, you can get it in different thicknesses and cut it to fit.

Make a simple mold using flashing and binder clips.
Fill it with epoxy based potting compound (the stuff used for electronics).

Make a block the approximate size needed but slightly larger.

After it hardens file it to fit.

Test it out and get it working the way you want.
Drill some 1/8 inch or so hoes in the mag and use epoxy to anchor it in place solidly.
Clean up excess squeeze out through the holes.

Thanks guys, the reason I was thinking of epoxy is because I don't have a mill and filling a piece of metal or even plastic that size by hand doesn't sound fun.

For now I'm going to hold off and buy a magazine from another manufacturer and go from there.  

The additives are what make the properties.

For stuff that matters, I use West Systems (although System 3 is great), and choose the additives that I want.  For something that that, I might consider milled fiberglass.  And if I want it to look good, you can add a colorant.  In a pinch, even something as simple as line chalk from the hardware store will look great, and since you're not trying to hit specific high-end strength-to-weight ratios in a laminate, it will be just fine.

As others have said, milling a piece of plastic might be a better choice, but if you can make a mold, doing it with epoxy would also work perfectly well.  But, by the time you get the epoxy, milled fiberglass, some chalk, and build the mold, you'll probably be into it more money and time than just using ABS, polycarbonate, whatever.  

You can do the rough milling of plastic quite quickly and easily with a rough file or a dremel, even finer-tooth hand saws if you're desperate.  If you just need a rectangular block, a plastics supplier would probably  have an appropriate width, and cut it for you for a few extra bucks.  I guess the ultimate decision depends on what kind of tools you have available.

Shoot, come to think of it, even if it were complex and you had no tools, you could just CAD out the design and have someone do it on a 3D printer for you.

So many options.

If you can get an oil film on the magazine and it is a convenient physical shape you could use it as a mold.
Put some modeling clay in the end you want to remain open.

Cast, pop it out, clean it up a little, decide on how you want to anchor it in.

You could use Apoxy.  It's a 2 part epoxy system that is like modeling clay.  Very convenient.  Can be wetted and smoothed.  Extremely useful stuff for building up surfaces/contours.