Actually Andy, I just drained water off the cover today in preparation to begin opening. My water temp at 2' depth is not quite 50 degrees right now. Possible snow flurries tomorrow.

I have the pieces ready to put my poly filter together, just haven't gotten to it yet. I'll have it ready for my first needed water top-off for sure. I like your drill bit tip. Sounds easy and not much trouble.
On a plus note, due to all the snow we got this year, my winter cover held enough iron-free water to completely top off my pool today with enough extra to complete my opening backwash. I simply duct taped a filter sock over my portable utility pump intake and another over the end of an attached 6' piece of garden hose, and transferred "double-filtered" (sorta) water under the cover and into the pool. When I pulled back my cover a few feet, my pool water was crystal clear, both before and after my addition. I'll get my first water sample shortly, and begin the TFP process for another season.
If you come across anything else to make your gadget better, let me know. Usually
I'm the guinea pig! LOL
Take care and be safe.