On my old boat, I inserted a radiator hose tee and cap, similar to this. Universal Radiator Hose Tee Flush Kit (47018) on eBay!
They have plungers with a hose fitting that you can attach to the hull at the thru-hull. Once the boat is out of the water, I have one that I've never used.
I do what you did, take the hose off after the strainer and use a large funnel to pour anti-freeze through, seems to work fine.