Hi
Im new at this, having made one uni-mould and one putty-mould before this.

i have a beaten plastic underside panel on my car that i was hoping to create a mould from.
As i have not been able to source a mint piece, this is what i have to work with.

Underside (facing away from the mould)





Side facing the mould



So i was thinking of taping and gluing it back together.
To cover the holes, where the plastic is gone i figured i could use some sort of backingplate, and fill it up with plasticine.

Any advice appreciated

I would hot glue some signboard behind the holes and then fill them with plastercine.

If you treated yourself to some of the heatproof/release tape you could simply fill the hole with cardboard or what ever and then release tape over that, the mould will release from this no problem.

while your at it use the signboard to make flanges and seal any holt holes with the release tape.