I have the same collars matter of fact that is the only collars I have all Magnum. Any way I get the 20" I think haven't had to buy any in a long time. I found that the tube for the magnets one size bigger than the tube for the antenna works best. But you don't have go get two sizes if you cut the strap for the magnet narrow .Don't no the exact size but 2 things, don't use the zip ties for the antenna it will cause the antenna to bend put pressure on the base and make the connection weak reducing signal range. Use the tube size closest to the 1inch, it should slide over the running end of the collar and antenna if it loose on the collar its to big it should slide over the collar but be snug. Cut it long enough to to support the base of the antenna and be close to the buckle when the collar is buckled on the last hole but not be in the way of the buckle some where around 2 to 3 inches give or take .
The magnet strap or tube for holding the magnet, I've seen where some cut the tube for the band around the case for holding the magnet around 1 inch to 1 1/2 to 2 inches to turn the collar off and hold the magnet.
From experience with Magnum collars, that's all I've ever used and haven't found anything to match or compare. I cut the inner tube for the magnet around a 1/4 to no more than 1/2 inch. This is more than enough to hold the magnet in place and has other benefits. The wider strap actually gets cut up and rips faster than the smaller narrow width ones and they last longer. You get more use from the tube.
I'm sure I'm not the only one that's turned Hounds out with the magnet still on the tracking collar. I've done it a time or two but I found out that the small strap will hold as good as the wide for storage between hunts, and if you have a brain laps and turn out with magnets in the collar the narrow strap will typically drop the magnet if the hound gets in a decent thicket ,versus the wide strap. The edge of the magnet is exposed so a briar thicket will catch the magnet pull it off and you can track the hound.
I cut the bike tube about half the length of the magnet and secure the magnet in the middle. This way it will hold the magnet in place securely while stored and if you have a brain laps leave the magnet on, usually
with-in a couple hours most of the time you can eventually track the hound. Hope this helps.
Just my 2 cents worth free of charge.

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