Okay p0g1.
Considering how good the comments have been I'll just say this.
1. Remember the iron man legs are disproportionate. they are longer then they should be. Why because the suit has raisers in the heel (think high heels), story wise it's because that's where the rockets are. So you have to decide wiether you want to stick with that, or modify the shin to actual length. (ie don't forget this when you scale the shin, use width as the length WILL be off).
So depending if you A: modify the leg or B: retain it's original length, how you do the boot will depend.
IF you go with A, then you can just take a pair of cheep boots and resin the parts right on and paint.
IF you go with B, well then here's my suggestion.
go into a cheep shoe place and hunt in the ladies yes ladies footware isle. look for a simple open top flat high heel shoe at least 2-3" high. make sure they fit. now take em home and while your foots in them, wrap some cloth over where your foot goes, not enough to trap your feet, but enough to cover up to the start of your ankle. This will help keep them on you, now resin all the fabric part of it to harden it up, now personally because i slush cast all my parts (pour resin in and rotate to get a even coat) when it's almost cured and is almost gel like, put the toe and heel covers onto the footwear, as the resin cures it WILL bond to both the part and the shoe, making a pretty perm bond that will be next to really really hard to seperate without destroying either. and voila, you got your boot.
Well that or you could just find yourself a pair of 70's style gogo boots.