On 31/08/2017 10:23, The Natural Philosopher wrote:
On 31/08/17 10:04, wrote:

You really need something that can be swollen to make a seal and get
some friction to hold against the pressure ,you will be lucky to find
a kids ball that will be just right.

Football bladder surely?

http://www.ebay.co.uk/itm/Cawila-Han...-/332182244155




Good idea. Plus a bag of spuds or woven shopping bag of garden soil,
placed on top , in case it slips.
I'm aware quite a lot of air pressure can build up , prior to the sewage
proper , getting anywhere near.
Because of marine flooding events , the usual air-balancing vents to the
manholes are absent, air-tight covers instead.