OK try this

vars/checkoutStep.groovy

@@ -8,9 +8,6 @@ void call(Map config = [:], Closure body) {
     echo 'Firing ...'
     runConfig = defaults + config
     stage(runConfig.stepName) {
-        repetitions = runConfig.stepName.length() + 2
-        boxTop = '+' + '=' * repetitions + '+'
-        echo "${boxTop}\n| ${runConfig.stepName} |\n${boxTop}\n"
         printBanner(runConfig.stepName)
         if (runConfig.useJobSCM) {
             checkout scm

vars/printBanner.groovy

@@ -1,5 +1,6 @@
 void printBanner(String anything) {
+    boxCorner = '+'
     repetitions = anything.length() + 2
-    boxTop = "+${'=' * repetitions}+"
+    boxTop = boxCorner + '=' * repetitions + boxCorner
     echo "${boxTop}\n| ${anything} |\n${boxTop}\n"
 }