double boiler is best way.
Depending on how many you have to do a simple way is to take a soup can and fill with wax, sit it in a pan of water and heat on the stove or hot eye.
Put you tube in the wax after it gets hot, let the tube warm up also. Put your finger over the vent hole, pull the tube out of the hot wax, place the bottom opening where the foundation and frame meet, and in one quick motion remove your finger from the vent hole and move the tube to the other end of the frame. This will take some practice to get the hang of it
One of the tricks is the temp of the wax, too hot will make a bigger mess and take longer to harden, too cool and it will not stick to the foundation and frame. Just have to play around with it.
good luck with it and don't get burned with the hot wax.
G3