No they will slide over the flexwatt fine no problem just use electrical tape over the cut 'strips' an over the connections I use foil tape alone just the edge to hold it down. Ranco and johnsons are designed to control heaters and units like oil filled heaters especially the cheapest versions that are with out complete thermostats of their own. Set it to max (or close to) and then allow the regular t-stat to regulate it.