Transferring InsolesFootbeds are typically transferrable providing they are not to long.
Take out the stock insole that is in the rental boot and line it up to your insole using the back of the heel as a reference point.
If your insole is shorter than the stock insole you will be OK as long as it isn't WAY short (1" or more). When the insole is short leverage inside the boot can cause the insole can shift for and aft. Some 2 sided tape or some duct tape reversed on itself might be necessary under the insole to keep it in place in the liner.
If the insole is to long it can ramp up in the toe box of the liner and give the impression the boot is to short. I can be trimmed or ground shorter but custom insoles are generally ground to a specific boot so beware. There are times when the radius of the toe of the insole is what doesn't match up. Usually you can grind or trim it to fit the liner better without permanently affecting the overall length of the insole.
Remember.....THE CUSTOM INSOLE REPLACES THE STOCK INSOLE. IT DOES NOT GO ON TOP OF IT.
If the boot still has to much volume shim it with flat stock (i.e. Bontex)