What kind of Creme or Mousse is the most robust? How can I strengthen a mousse?
There are several major types of mousse, made from different bases, and with different flavor elements.

Depending on which one you are using, they may have varying requirements.  As MandoMando mentions in the comments, assuming you are using a mousse based on whipped cream or whipped egg whites or a freezer-stable thickener (neither gelatin, agar agar, nor carageenan are freeze-stable), you can simply freeze the cake for transport.  This tends to work very well, although you may have issues with condensation when thawing it, so you want it very well wrapped (usually after freezing, so as not to muss the icing).

Chocolate mousses are often fairly stable due to the cocoa butter, which is solid at room temperature; some can also be gelatin stablized.  See the linked recipe in

Chocolate mousse - methods to have firm foamy mousse using white egg and cream

for inspiration, where the confection is built with two layers of mousse, one which is stablized with gelatin.

Many fruit mousses tend to be made with gelatin, so increasing the quantity of gelatin will give you a stronger mousse.

See also:
Hydro-colloid primer for perhaps more information on hydro-colloids than you ever wished to know.