I haven't done this myself, but had considered when I returned to larping getting a latex shield, one of the standard ones that are well advertised out there. I didn't like the mounts either and from what I planned I suggest carefully cutting away at the mass of material till its smooth. Then I'd punch holes to mound a handle where its most comfortable. For the upper arm strap, that's easy, but again, I'd punch a few holes and rivet, or use a nut and bolt to secure the strapping. I used that on my kydex shield a black leather belt (a cheap one, you see the venders in NYC all the time). The key though is cutting then shaving down the excess latex/foam on their mount well enough and not overdoing it where it impacts the integrity of the shield.
Good luck!
MC
OOG name: Mitch
Master Runesmith of Pendarvin
Don't let the grey hair fool you...wait scratch that...DO let the grey hair fool you.
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I make leather armor and various items for sca, larps and dagorhir. Here's my photobucket:
s297.photobucket.com/albums/mm233/mc1csmo/
