The easiest thing to do would be buy her a new home (pref a wooden viv) as you say she is fully grown im guessing she is around the 4/5ft mark? if so the 20gal tank she is in now is too small, in general the viv should be the lengh by the width = the lengh of the snake, (eg, lets say she's 5ft the viv will need to be 3ft long and 2 ft wide =5ft or 4ft wide and 1ft width =5ft (you get the picture) then get some good ventalation in there (hard to acheive in a glass viv) the best way to acheive this is to have ventalation holes at the bottom one end and at the top the other this way you will create an air cycle, cool air coming in from the bottom warming as it goes around the viv and exiting through the top ventalation holes, also make sure she has hide spots both in the cool and warm end of the viv or you may cause her just to sit either in the cool or warm end, if you can get the viv stable with just the heat lamp then there is no real need for the heat mat but could be placed under the viv and turned on after feeding to help her digest the meal more easily. hope this helps a little.