There could be a way of welding HDPE without spending money on buying expensive equipment... you'll need a thin strand of wire and a battery or power supply of some sort... I had experimented using this method some time back with very good results.

In simple words I put a thin strand of wire covering the weld area, then connected it to a battery which heated the wire up and fused to HDPE together, a 10mm thick sheet welded at the ends could take my weight on it without breaking...

Have a look here if you need details:
http://www.fightingrobots.co.uk/thre...ld-diary/page5

PS: the battery might not like this