I used to just use a red wire nut on caulk tubes and similar. However if you have an Ace Hardware around, see if they have the "Little Red Caps". These things do good, and have other uses too. My biggest problem is remembering where I put the package of them.
I stick a long screw or nail in the tip and cover it with w few wraps of painters tape. I also cover the bottom of the tube with painters tape. It will usually keep for a couple of months.
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2016 Wildwood 32BHDS
2004 F-250 CC 4X4 V10
Yep, I cover the both ends of the tube with duct tape and place the tube in the fridge in my shop. Yes I do have to wait for the tube to warm up before using again and usually overnight is enough.