To reiterate a point: If the OP has a half-inch play in the seatpost and uses epoxy to fill the gap, I suspect it will fail. In this scenario, the glue doesn't act to quickly distribute force to the metal structrural members, it becomes a structural member itself and will have to absorb normal frame stress. As Mad Honk points out, there already was a failure of aluminum. Had that aluminum been properly heat treated, it would have strength figures around 40,000 psi. If it wasn't properly treated, around 12,000 psi. And it broke. Epoxy (strength figures around 3000 psi) will surely fail.
If the OP finds a post that is a close fit, then the epoxy transfers stress and distributes it to the stronger aluminum frame elements. Your mileage may vary, and the consequences of trying this (I suggested a new frame) are on you, but a close-fitting (not too tight, but certainly not rattling around) post expoxied into the frame would likely work.
I use an industrial quality flexible epoxy (West Systems G-Flex) to get some toughness in joints. Here is a case where I'd probably not use Harbor Freight's epoxy. BTW, if the best post that can be found still presents a little gap, you can add fillers to epoxy to increase its bulk strength a bit (e.g. West Systems High Density Adhesive Filler). This stuff is all not super cheap, but it probably gives you the best chance of success. Remember to clean the surface per instructions.