thanks jonathan, what i was going to do is make a long enough rod with a 95mm x 95mm plate at the end so it fits in nice on my 100mm box sections and when pouring sand inside i just wack the plate so compacting the sand as tight as possible and then just bolt some plates at the end so no sand can escape. i think this idea would be a much better result then just pouring sand in do you think.

cheers

Ash