I know the question has been asked before but I am still having issues over whether to use back or belly heat

How does everyone deal with the change in height when rolling their tubs over a heating pad? Do you have to have a little gap above the tub and ceiling?

If I use back heat this will pose no problem.....however I would prefer a heating pad underneath

I thought of using thin plastic strips to act as runners for the tub,

what you guys think?