If I recall correctly they had to adapt square CO2 scrubbers into round holes with sticky tape etc...I Imagine a knife would have been used useful.
I don't think that a knife was used to cut the duct tape on Apollo 13. One of the properties of duct tape is that it is easily torn maintaining a straight line due to the nature of its construction.
Below is a portion of the steps sent up to the crew to construct and adapt the CO2 canister to fit the round hole. See step #2.
PROCEEDURE - 19 steps
#1) Place the LCG bag over the top of the square lithium-hydroxide canister. The bag must be pulled down to just over the triangular ventilator slots on the side.
NOTE: Be careful not to rip the bag because there are only three bags onboard.
#2) Tear the duct tape down the middle lengthwise to double the linear length.
#3) Seal the bag to the square canister by wrapping the duct tape around the canister where the bag opening is.
NOTE: The tape must seal the opening to prevent leakage of air flow.
#4) Poke or cut a hole in the middle of the top of the covering bag approximately the diameter of the hose from the red suit.
#5) Insert the hose into the hole. Secure the hose connection into the LCG bag with duct tape.
Also, if something did need to be cut most likely the crew would have used a pair of Weck surgical scissors carried by all the Apollo crews as seen below.
