Sounds like you only have One bow roller? Most had two rollers on a toggle, one below the bow eye, and one above. I hated that setup because the bow eye would catch on the lower roller. I replaced the toggle with one fixed roller above the bow eye.
I keep my H240 bow eye tight up to the roller (or within 1 inch), and have never had it slide, even without hold-down straps. However, I did replace the winch strap with a 10,000 lb rated strap, not the wimpy strap that was on there when I bought it used (seat belt size strap). The little strap was scarry tight, but the 10,000 lb strap does great. The two speed winch was plenty strong enough.
Here is a picture of my setup, shown prior to adding the heavier strap:
Here is a picture of Jdpber's setup, which is similar, but with a hold down turnbuckle, and the lighter strap:
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