Easiest and cheapest way: grab some 2" PVC, cut to length you need,
then use a heat gun to soften it up. It becomes quite elastic: you can
expand it around 30-40%. Use the 2.5" piece to force it in and wiggle
around until the elasticized PVC adapts to the correct size. Use gloves
to handle it, and do it outside to avoid any fumes from the plastics.

Around $2 for an 8' length of PVC = 24, 4" thick and sturdy adapters
or $6-$7 each for the Borg authorized adaptors.

Your choice,
H