The pipe is the mould. With what I understand of your current system, when you set the up the infusion you would probably end up with resin running up the join in the pipe instead of through the fibres. This would result in a lot of dry areas. Having said that it would be relatively easy to set up your bag so the resin would go in one gap, through the fibres and then out the other gab. I might do an experiment at some stage.

There are spray adhesives that are used to hold fibres in location while setting up.