I appreciate this is an old thread now - but this deals with the same quandary I'm facing at the moment. I think I'm going to run with the Vango tarp as a stop-gap for this season and use it the way you have. Can I ask - is the front pole extendable, or the one which came with the tarp?
Also does the rig only work with the side door shelter, or have you tried it on the front of the tent?
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Just wondered how you got on with the Vango tarp on the Montana? We tried it but found the Vango tarp was just too small, as the front of the Montana is quite high.
Then we tried the Decathlon Quechua adventure tarp which has served well, but last weekend the weather was both windy and rainy and it blew down.
The downside with the Montana as all owners know is the leaky front door, and I would really like to solve it as am fed up with puddles in the doorway and wet shoes. So am trying to persuade Hubby that we should get the Montana extension.... cheapest I have found it is £130. Go Outdoors have them but only in certain stores up north.