Both woffers are the saame, both get the same signal. Ask Simply Speakers, Orange County Speakers, or the like what size of dust cap will fit.
Be careful, as the cones can be brittle and crack. A steady hand and a sharp Xacto knife will be needed.
If you already have the woffers otu, I would think about adding some damping material ti the back-PArts express has some nice choices. It looks like the stock piece is about 3/4 thick, so I would go with the 3/4 thick stuff on the back panel. I woudn't add too much, or you will reduce the internal volume. Perhaps the Crites diaphrams as well.
I have always thought the KG 5.5 was a fairly uncolored Klipsch that has great dynamics, and have enjoyed them from the day I bought them new.
I have avoided any tinkering to date, as I don't want to change the sound for the worse, however looking at those who swear by the Crites TI diaphrams will lead me to this in the future. At that point I will go with the damping on the back panel. This makes more sense than the stock foam piece wedged behind the woofer, and should in theory help the midbass.
Interesting side note, Paul Klipsch apparently liked the sound and the resonace of plywood enclosures