If you want to fit a budget then I recommend buying knockoff clones off amazon/ebay/aliexpress since there are some actually good quality knockoffs (such as this: https://www.amazon.com/dp/B098BL62YH/ref=cm_sw_r_cp_api_i_R9P3R178E2XMMRXA9JS3?_encoding=UTF8&psc=1) and swapping the helmets with custom/real helmets bought off ebay (i used this seller for helmets: https://www.ebay.com/str/nashvegasbricks) My army includes some custom and real clones too which drove the price up since customs were about $20-30 each and there are more than 10 here. I didn’t track the total price tho lol
Instead of Clay, try green stuff maybe.
I've never used it personally, but I've seen people use it for small things.
I'm not sure if it'll work with something big like a helmet, but I don't think it would hurt to try it out.